CPStringBuilder sounds suspicious. The slowdown is due to loads of extra allocation, mostly of int arrays, char arrays, and regular expression related junk. While there is a 4-5 fold increase in the number of gcs, the total gc time increase is minimal, suggesting that most of the allocated objects are collected by the nursery ...
The types with the 5 largest number of allocators on luindex before the change:
I have attached full runs with more statistics.