The Play Mode that took six minutes
Early in my time at Unity, a studio reported that entering Play Mode had gone from about 10–15 seconds to over a minute and a half after upgrading from 2018.4 to 2019.4 — bad enough to block their whole team’s engine upgrade ahead of a launch. On my machine their project was even worse: the domain reload took 362 seconds. The usual suspects (platform settings, asset pipeline rollbacks, the generic checklist) changed nothing, so I went digging with the Editor Iteration Profiler and then further down: I modified the Editor source to print per-object timings for the managed-object backup/restore step of the domain reload, dumped all ~4,800 objects into a table, and sorted it. One single component — a debug overlay — accounted for 91% of the restore time.
The root cause was a serialized array of objects whose type contained nested lists of itself: an object-composition cycle serialized by value. Unity’s serialization depth limit had quietly risen from 7 to 10 between those two versions, and each extra level multiplied the object graph — the Editor log had even been warning about it (“Serialization depth limit exceeded”) all along. Removing the self-referencing fields dropped the reload from 362 to 4.6 seconds, faster than the version they were upgrading from. Two lessons stuck with me: a “performance regression after upgrade” is often an old defect that a new version stopped forgiving, and the boring warning everyone scrolls past is frequently the whole answer. It remains one of my favorite tickets — my manager still owed the team drinks over it.