That's a very good point, but merging in three years of changes would also have pulled in a lot of minor performance changes, and possibly some incompatible changes that would require some code updates. That would slow down each bisection step, and also make it harder to pinpoint the problem.
If you know that some small set of recent changes caused a 7% regression, you can be fairly confident there's a single cause. If 3 years of updates cause a 6% or 8% regression (say), it's not obvious that there's going to be a single cause. Even if you find a commit that looks bad, it might already have been addressed in a later commit.
Edit to clarify: you're technically correct (the best kind of correct!) but I'd still much prefer to merge 3 weeks rather than 3 years, even though their justification isn't quite right.
If you know that some small set of recent changes caused a 7% regression, you can be fairly confident there's a single cause. If 3 years of updates cause a 6% or 8% regression (say), it's not obvious that there's going to be a single cause. Even if you find a commit that looks bad, it might already have been addressed in a later commit.
Edit to clarify: you're technically correct (the best kind of correct!) but I'd still much prefer to merge 3 weeks rather than 3 years, even though their justification isn't quite right.