Problem/Motivation
#2694391: Separate cache bin for migrations removed some of the discovery caching for migrations to speed up development.
While this should be negligible for migrations run via drush, we don't know the impact on migrations run via the UI.
In general, the performance of migrate batch requests is worth looking at in general - we might find migrate performance issues here that are harder to see via drush - and given the number of requests made per migration a hundred milliseconds could add up to several seconds or minutes by the time the migration is done.
Proposed resolution
Run a migration from 6-8 and profile the migrate batch request with xhprof or similar.