There were many performance changes after Bug 1702511:

• alert: https://treeherder.mozilla.org/perfherder/alerts?id=30274&hideDwnToInv=0
• pushlog: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=b021a3ea639f8ffae79fb593c12f9142272edfbb&tochange=391dbe0ceb290de3c1a6989aab62e602e55f176c

Summary of regressions:

• custom.webconsole.open +200% regression but only on linux. It looks like other platforms were either already at 2000ms for this test, or were oscillating between 2 values. (graphs) Meaning this change consolidated the baseline at 2000ms and might not be a pure regression.
• webconsole.openwithcache +100% regression on linux, smaller regression on other platforms (graphs)
• complicated.webconsole.open +35% on all platforms (graphs)
  Simple.webconsole.open +30% on all platforms (graphs)
• console.bulklog +12%
• simple.jsdebugger.reload regression (lost the improvement from last week)
• cold.webconsole.open +20-70% regression

Some improvements

• console.typing -4-10%
• screenshot -10%

Pushed to try to check if the regression is actually linked to Bug 1702511:
https://treeherder.mozilla.org/perfherder/compare?originalProject=try&originalRevision=9bf1933a7a8a8f6765a26788a134a87c72da0233&newProject=try&newRevision=ee38ce0d0eb8141d82f3e5de4834c96c4c80df0a&framework=12

DAMP results are available on the link above and confirm that the changes are linked to Bug 1702511.