Old discussion:  http://crbug.com/483354 

Created a test to measure the performance improvements objectively: http://crrev.com/1920783002

Confirmed the Android/470ms "sadness" of "main resource fetch with a cold Service Worker" by a test[1]. In my environment (Nexus7 2013, Release build of today's master), in-scope iframe creation with cold sw/hot sw took around 100ms/40ms while out-of-scope took 10-20ms.

[1] https://github.com/amiq11/Service-Worker-Performance/commit/d45228bf47ca4ca04ef082ad2bcc7786c4dc874a

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb

commit ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb
Author: shimazu <shimazu@chromium.org>
Date: Fri May 27 04:34:04 2016

ServiceWorker: Store the existence of fetch event handler

Currently |has_fetch_handler| is always set to true though the ServiceWorker script has no fetch event handler.
This patch passes the existence of fetch handler in the response to the install event, so the correct value will be stored.
This information will be used for bypassing ServiceWorker when no fetch handler is registered.

BUG= 605844 
TEST=./out/Default/content_unittests  --gtest_filter="ServiceWorkerJobTest.HasFetchHandler"
TEST=./out/Default/content_browsertests --gtest_filter="ServiceWorkerVersionBrowserTest.InstallWith*"

Review-Url: https://codereview.chromium.org/2002883002
Cr-Commit-Position: refs/heads/master@{#396381}

[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/content/browser/service_worker/embedded_worker_test_helper.cc
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/content/browser/service_worker/service_worker_browsertest.cc
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/content/browser/service_worker/service_worker_context_unittest.cc
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/content/browser/service_worker/service_worker_job_unittest.cc
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/content/browser/service_worker/service_worker_register_job.cc
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/content/browser/service_worker/service_worker_register_job.h
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/content/browser/service_worker/service_worker_storage.cc
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/content/browser/service_worker/service_worker_version.h
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/content/common/service_worker/service_worker_messages.h
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/content/renderer/service_worker/service_worker_context_client.cc
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/third_party/WebKit/Source/web/ServiceWorkerGlobalScopeProxy.cpp
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/third_party/WebKit/Source/web/ServiceWorkerGlobalScopeProxy.h
[modify] https://crrev.com/ceec8e3a24f7ad3a2bd7a64ae7f6d82f25dfabdb/third_party/WebKit/public/web/modules/serviceworker/WebServiceWorkerContextProxy.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8a5ede2ab476d57dbce99422d89333b71cd7dcab

commit 8a5ede2ab476d57dbce99422d89333b71cd7dcab
Author: shimazu <shimazu@chromium.org>
Date: Tue May 31 05:28:42 2016

ServiceWorker: Bypass SW when the script doesn't have fetch handler

When the script doesn't have a fetch handler, a request
will immediately go to the network after passed to SW in current implementation.
However, if SW is down, this procedure will wait for waking SW up though SW
doesn't have fetch handler. This patch enables the request to directly go to
network bypassing SW in this situation.

BUG= 605844 
TEST=/out/Release/content_unittest --gtest_filter="ServiceWorkerControlleeRequestHandlerTest.FallbackWithNoFetchHandler"

Review-Url: https://codereview.chromium.org/2019613003
Cr-Commit-Position: refs/heads/master@{#396787}

[modify] https://crrev.com/8a5ede2ab476d57dbce99422d89333b71cd7dcab/content/browser/service_worker/service_worker_controllee_request_handler.cc
[modify] https://crrev.com/8a5ede2ab476d57dbce99422d89333b71cd7dcab/content/browser/service_worker/service_worker_controllee_request_handler_unittest.cc
[modify] https://crrev.com/8a5ede2ab476d57dbce99422d89333b71cd7dcab/content/browser/service_worker/service_worker_version.h

Just out of curiosity, are you still checking for service worker updates even if the fetch event is not fired?

I wrote a spec issue for this, but was wondering what you were implementing to start:

https://github.com/slightlyoff/ServiceWorker/issues/905

Yes, updates are checked as usual. Actually we should add an assertion for that in our test (shimazu@ originally had one which I wrongly asked him to remove).

I'll comment on the spec issue too.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/db1d53558883c7301360ddb05f0895fe05ae3408

commit db1d53558883c7301360ddb05f0895fe05ae3408
Author: shimazu <shimazu@chromium.org>
Date: Wed Jun 01 04:54:47 2016

ServiceWorker: Distinguish SW.PageLoad w/ and w/o fetch handler

This patch separates ServiceWorker.PageLoad for serviceworker into the two: w/
and w/o fetch handler in order to measure the impact of the no-fetch handler
optimization.

BUG= 605844 

Review-Url: https://codereview.chromium.org/2025813002
Cr-Commit-Position: refs/heads/master@{#397060}

[modify] https://crrev.com/db1d53558883c7301360ddb05f0895fe05ae3408/content/browser/service_worker/service_worker_controllee_request_handler.cc
[modify] https://crrev.com/db1d53558883c7301360ddb05f0895fe05ae3408/content/browser/service_worker/service_worker_metrics.cc
[modify] https://crrev.com/db1d53558883c7301360ddb05f0895fe05ae3408/content/browser/service_worker/service_worker_metrics.h
[modify] https://crrev.com/db1d53558883c7301360ddb05f0895fe05ae3408/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c367daaa701cf3e439ab3f75108a2fdd2c00342b

commit c367daaa701cf3e439ab3f75108a2fdd2c00342b
Author: shimazu <shimazu@chromium.org>
Date: Tue Jun 21 09:14:43 2016

Revert of ServiceWorker: Bypass SW when the script doesn't have fetch handler (patchset #5 id:80001 of https://codereview.chromium.org/2019613003/ )

Reason for revert:
See:  http://crbug.com/621788 

Original issue's description:
> ServiceWorker: Bypass SW when the script doesn't have fetch handler
>
> When the script doesn't have a fetch handler, a request
> will immediately go to the network after passed to SW in current implementation.
> However, if SW is down, this procedure will wait for waking SW up though SW
> doesn't have fetch handler. This patch enables the request to directly go to
> network bypassing SW in this situation.
>
> BUG= 605844 
> TEST=/out/Release/content_unittest --gtest_filter="ServiceWorkerControlleeRequestHandlerTest.FallbackWithNoFetchHandler"
>
> Committed: https://crrev.com/8a5ede2ab476d57dbce99422d89333b71cd7dcab
> Cr-Commit-Position: refs/heads/master@{#396787}

TBR=falken@chromium.org,mek@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG= 605844 

Review-Url: https://codereview.chromium.org/2080793003
Cr-Commit-Position: refs/heads/master@{#400928}

[modify] https://crrev.com/c367daaa701cf3e439ab3f75108a2fdd2c00342b/content/browser/service_worker/service_worker_controllee_request_handler.cc
[modify] https://crrev.com/c367daaa701cf3e439ab3f75108a2fdd2c00342b/content/browser/service_worker/service_worker_controllee_request_handler_unittest.cc
[modify] https://crrev.com/c367daaa701cf3e439ab3f75108a2fdd2c00342b/content/browser/service_worker/service_worker_version.h

Reopened due to  http://crbug.com/621788 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e14963a7be1f6445401efb43619301bf9fe3157d

commit e14963a7be1f6445401efb43619301bf9fe3157d
Author: shimazu <shimazu@chromium.org>
Date: Thu Jul 07 09:31:48 2016

ServiceWorker: Add an API to fallback to renderer for CORS preflight

Currently ServiceWorkerURLRequestJob::ForwardToNetwork invokes restart
of the request and the request goes to the network directly. In
subresource case, this causes an bug when the request needs CORS
preflight.
This patch adds an API to fallback with CORS check for subresources; this
is used at a subsequent patch: aka no-fetch optimization
http://crrev.com/2103063002.

BUG= 621788 , 605844 

Review-Url: https://codereview.chromium.org/2108573002
Cr-Commit-Position: refs/heads/master@{#404114}

[modify] https://crrev.com/e14963a7be1f6445401efb43619301bf9fe3157d/content/browser/service_worker/service_worker_url_request_job.cc
[modify] https://crrev.com/e14963a7be1f6445401efb43619301bf9fe3157d/content/browser/service_worker/service_worker_url_request_job.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cb53dfa21239d8be2ed3bc53c83d8c001603cee6

commit cb53dfa21239d8be2ed3bc53c83d8c001603cee6
Author: shimazu <shimazu@chromium.org>
Date: Fri Jul 08 14:04:11 2016

ServiceWorker: Reland of bypassing SW when no fetch handler existed

When the script doesn't have fetch handler, in-scope network requests
don't need to go to the service worker. This patch is to bypass it.
This patch is a CL for re-landing which was reverted at
http://crrev.com/2080793003.

This CL should be landed after http://crrev.com/2108573002 is committed.

BUG= 605844 , 621788 
TEST=./out/Debug/content_unittests --gtest_filter="ServiceWorkerControlleeRequestHandlerTest.FallbackWithNoFetchHandler*"

Review-Url: https://codereview.chromium.org/2103063002
Cr-Commit-Position: refs/heads/master@{#404348}

[modify] https://crrev.com/cb53dfa21239d8be2ed3bc53c83d8c001603cee6/content/browser/service_worker/service_worker_controllee_request_handler.cc
[modify] https://crrev.com/cb53dfa21239d8be2ed3bc53c83d8c001603cee6/content/browser/service_worker/service_worker_controllee_request_handler_unittest.cc
[modify] https://crrev.com/cb53dfa21239d8be2ed3bc53c83d8c001603cee6/content/browser/service_worker/service_worker_version.h

This is in M54.