This resembles a situation similar to bug 1525841. It happened around February 11. Doing retriggers on older commits produced values equals to those from the "regression"/"improvement". Thus, this sounds like an infra change.

Regressions:

5% Base Content Resident Unique Memory windows10-64 pgo stylo 14,652,074.67 -> 15,347,200.00

Improvements:

5% Resident Memory windows10-64-qr opt stylo 545,113,937.61 -> 520,289,049.45

You can find links to graphs and comparison views for each of the above tests at: https://treeherder.mozilla.org/perf.html#/alerts?id=19292

Rob, can you confirm this?

This seems to have changed the Base Content Resident Unique Memory baseline on mozilla-beta as well.

the timing coincides with the upgrade from windows 10 build 1703 to windows 10 build 1803 in bug 1522896