Thank you for finding this @jdescottes].
I haven't run the profile yet but afaict finding declaration issues is computation intensive process because of the cross checking against the CSS dataset and then the browser dataset to find the compatibility issues so https://searchfox.org/mozilla-central/rev/6cc48251bb97600fdf11a5b4c5f621bfc8606d55/devtools/server/actors/styles.js#1680-1682 might cause slowdown.
Most of tooltip operations are async but there is a O(n) check that happens against the compatibility issues to display the tooltip icon. This check can be found here
For m rules (most being compatible), the ruleview will take O(mn) time to compute the visibility of tooltip icon. (This check will not happen if the pref is disabled however but the compatibility issues will still be calculated in the StyleRuleActor. If this is happening despite the pref staying disabled, then the form creation might be the cause of the perf regression)
If it is related to the two bugs mentioned then it would be one of these that is causing the regression.
If it is the former where the form is taking a long time to be created, Patch D87613 (https://phabricator.services.mozilla.com/D87613) will indirectly address this as we look to remove the association of compatibility issues with the style rule actor and instead connect to the compatibility actor to get the data. If this is the cause, as an immediate solution, we can only perform the compatibility issue computation if the pref is enabled.
Please let us know if you find the root cause for this performance regression. I'll also look into this in the meantime.