IMO the time differences you are seeing can't be explained by slowdown due to threading. Using a shorter test string "aibicid", I do see some differences in the output generated by use re "debug"; between 5.6.x and 5.8.x, but don't know enough to interpret the output.

I'd encourage you to report this as a bug. Even if (as I expect will be the case) you are then told that the slowdown is due to necessary bug fixes, someone may come up with a better way.