The best way to track it down would be bisecting. Take a good version and a bad version, and set:
git bisect start
git bisect bad <bad-commit-sha1>
git bisect good <bad-commit-sha1>
It will then do a binary search to find the first commit that caused performance to be worse. After each trial, just tell it the result:
qmake
<test speed on phone>
git bisect good
qmake
<test speed on phone>
git bisect bad
If there are 1000 commits, this will take approximately 10 tries to find the exact cause (more for multiple causes, though, of course.)
-[Unknown]
git bisect start
git bisect bad <bad-commit-sha1>
git bisect good <bad-commit-sha1>
It will then do a binary search to find the first commit that caused performance to be worse. After each trial, just tell it the result:
qmake
<test speed on phone>
git bisect good
qmake
<test speed on phone>
git bisect bad
If there are 1000 commits, this will take approximately 10 tries to find the exact cause (more for multiple causes, though, of course.)
-[Unknown]

