I am able to reproduce the difference in load time for the "manage people" window.

At first glance, it seems that Polymer2 is running a very slow Regex within replaceDirInCssText() that takes 700ms, which in turn is called once for every call to Polymer() function. 

 Will investigate further, since I still don't  have a good explanation of why this only happens for md-user-manager, but suspecting that is because in other Polymerized WebUI pages we don't use CSS files anymore in favor of <style> modules.

Attaching trace.

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FYI, verified that replaceDirInCssText() is the cause of the slowness. If I comment it out within Polymer itself, the page loads very fast (~500ms), see attachment.

Note that md-user-manager includes several inlined PNG images in its CSS, which I believe is why it take so long for the Regex within replaceDirInCssText() to process it (unlike Settings or other pages).

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we don't currently use :dir() in webui's code (AFAICT) and I'd be happy to create a patch to remove this in the meantime (w/ a presubmit that disables folks from mistakenly adding uses).

as i mentioned to dpapad@ over chat: if we get away from the legacy Polymer({...}) syntax, I think we'd have more control / ability to opt into this mixin (right now it's hard to opt out).  That's still a ways away, though.

WIP CL https://chromium-review.googlesource.com/c/chromium/src/+/1306515, which removes any usage of DirMixin from the final polymer2 bundle under third_party/polymer.

After inspecting the code found two usages of :dir().

1) In https://cs.chromium.org/chromium/src/third_party/polymer/v1_0/components-chromium/paper-input/paper-input-char-counter.html?l=46, which can be removed because this element is not used anymore (last usage from paper-input is removed at https://chromium-review.googlesource.com/c/chromium/src/+/1306346)

2) In https://cs.chromium.org/chromium/src/third_party/polymer/v1_0/components-chromium/paper-input/paper-input-container.html?l=257,258. It's still not clear to me whether this makes use of the :dir() rule, will investigate further. Either way, this is supposed to be migrated to cr-input tracked by issue 856310.

I'll remove 1 first, and then investigate 2 a bit more. After both of these are removed, it seems safe to go with the CL at #6 to dir-mixin.html from Polymer 2. 

Re #8:
1) is being addressed at https://chromium-review.googlesource.com/c/chromium/src/+/1309226.

2) I posted the wrong link previously. The Chromium usage is at https://cs.chromium.org/chromium/src/chrome/browser/resources/chromeos/login/gaia_input.html?l=8.

 Issue 900543  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9d00b88f1ffbb8219bf78ff12aa1077b7e16af15

commit 9d00b88f1ffbb8219bf78ff12aa1077b7e16af15
Author: dpapad <dpapad@chromium.org>
Date: Thu Nov 01 23:51:29 2018

WebUI Polymer 2: Don't include dir-mixin.html in the final bundle.

Locally modify Polymer 2 to exclude dir-mixin.html, since it is unnecessary
currently (we don't directly* use any :dir() CSS rules), and also causes a
performance regression (more notice-able in chrome://md-user-manager), due to a
Regex running over a fairly large amount of text (because of inlined PNG background
images).

*The only usage of :dir() was in paper-input-container.html, which is still used
by ChromeOS <gaia-input> element. Modified <paper-input-container> to directly
use :host-context instead. Also :dir() is not implemented in Chrome, so we should
not be using it in our own code anyway.

Ideally the usage of this mixin should be exposed as a Polymer option, so that
we don't need to manually modify our local Polymer 2 copy.

Bug:  899603 
Change-Id: Ia20535fb2adb2c9a6bebf3046e4405a4ca31af6c
Reviewed-on: https://chromium-review.googlesource.com/c/1306515
Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604759}
[modify] https://crrev.com/9d00b88f1ffbb8219bf78ff12aa1077b7e16af15/third_party/polymer/README.chromium
[modify] https://crrev.com/9d00b88f1ffbb8219bf78ff12aa1077b7e16af15/third_party/polymer/v1_0/chromium.patch
[modify] https://crrev.com/9d00b88f1ffbb8219bf78ff12aa1077b7e16af15/third_party/polymer/v1_0/components-chromium/paper-input/paper-input-container.html
[modify] https://crrev.com/9d00b88f1ffbb8219bf78ff12aa1077b7e16af15/third_party/polymer/v1_0/components-chromium/polymer2/polymer-extracted.js
[modify] https://crrev.com/9d00b88f1ffbb8219bf78ff12aa1077b7e16af15/third_party/polymer/v1_0/reproduce.sh

Update :

Rechecked the above issue on Windows(7,8,8.1,10), Mac(10.13.1 , 10.13.6, 10.14.1) and Linux(14.04 LTS)OS with latest Canary version #72.0.3599.0 and the issue is fixed.

Kindly refer the attached screen cast.

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