Originally reported here: https://github.com/firefox-devtools/debugger/issues/7806

The feature Map Scopes currently has a significant performance cost for the parser worker and non-trivial cost for the main thread.

w/ map scopes on -- http://bit.ly/2Tf9KqC
w/ map scopes off -- http://bit.ly/2Tg4aUL

The reason for that is that parsing the generated file is often slow, building scopes list can be slow, and building generated lists is about ~60ms on the main thread for debugger.html

We can make map scopes opt-in and let users choose to have the feature. This could be good for several reasons:

1. users will know that the feature exists :)
2. users will know that we want feedback, more likely to send us reports if mapping fails or is slow
3. users can keep it off if they want speed

While investigating a few issues where certain variables show undefined in the tooltip previews, most of the issues related to the fact that Map Scopes was not turned on. This makes it clear that this feature is not quite visible to users.

This brings up a couple of options

1. Map scopes is known to be have significant performance cost (See comment 0) so its switched off be default. Could we see if we can improve it and switch on be default (IDEAL)?
2. Improve the UX by making it more discoverable, and/or adding guides to help users understand why they have the related issues and how to turn it on if need be.

Map Scopes currently affects the previews, watch expressions, console, Scopes panel.

Comment on attachment 9342300 [details]
WIP: Bug 1695621 - [devtools] Turn on map scopes for damp

Revision D182758 was moved to bug 1844083. Setting attachment 9342300 [details] to obsolete.