This is already included in the request, just zlib compressed and base64 encoded.  I think a better approach would be able to decode this client side, create the shortened version and have the number of items and size and send that instead of the compressed/encoded bit.

This code has to be translated into javascript, including zlib decompression (which can be done with pako):
https://chromium.googlesource.com/infra/luci/luci-py/+/master/client/utils/large.py#46

It'd be useful to diagnose overhead.

To get this data, the request must include &include_performance_stats=true

Showing totals for each items_cold / items_hot in both number of files and total sizes; then add the full list on hover or something but it's really the totals that are generally useful.

bpastene, jbudorick, fyi ...

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-py.git/+/f9b80e13eb1b189c52eba5dc757d2cd98d52a9f7

commit f9b80e13eb1b189c52eba5dc757d2cd98d52a9f7
Author: Marc-Antoine Ruel <maruel@chromium.org>
Date: Thu Aug 10 17:20:45 2017

swarming: add precalculated values for cache hit rate.

This removes the need from decompressing the flate'd stream just
to get the number of items and the sum.

Edit test expectations to add more variance.

R=kjlubick@chromium.org
BUG= 672090 

Change-Id: I120dffb7a0d2c536027f081fa43db1e67653c4af
Reviewed-on: https://chromium-review.googlesource.com/610442
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>
Reviewed-by: Kevin Lubick <kjlubick@chromium.org>

[modify] https://crrev.com/f9b80e13eb1b189c52eba5dc757d2cd98d52a9f7/appengine/swarming/handlers_endpoints_test.py
[modify] https://crrev.com/f9b80e13eb1b189c52eba5dc757d2cd98d52a9f7/appengine/swarming/local_smoke_test.py
[modify] https://crrev.com/f9b80e13eb1b189c52eba5dc757d2cd98d52a9f7/appengine/swarming/server/task_result.py
[modify] https://crrev.com/f9b80e13eb1b189c52eba5dc757d2cd98d52a9f7/appengine/swarming/server/task_result_test.py
[modify] https://crrev.com/f9b80e13eb1b189c52eba5dc757d2cd98d52a9f7/appengine/swarming/swarming_rpcs.py
[modify] https://crrev.com/f9b80e13eb1b189c52eba5dc757d2cd98d52a9f7/appengine/swarming/test_env_handlers.py

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-py.git/+/5561f051f72027fc96deb938c57515096c82472e

commit 5561f051f72027fc96deb938c57515096c82472e
Author: Kevin Lubick <kjlubick@google.com>
Date: Fri Aug 11 12:03:38 2017

Add cache summary to Swarming UI

Bug:672090
Change-Id: Ie416c8d77d8e36995e9dfb5859c9ba18eb9a96af
Reviewed-on: https://chromium-review.googlesource.com/611023
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Commit-Queue: Kevin Lubick <kjlubick@chromium.org>

[modify] https://crrev.com/5561f051f72027fc96deb938c57515096c82472e/appengine/swarming/ui/build/elements.html
[modify] https://crrev.com/5561f051f72027fc96deb938c57515096c82472e/appengine/swarming/ui/res/imp/taskpage/task-page.html

This is live in chrome-swarm and chromium-swarming