This may be unrelated to this pilot, but since Spring 19 arrived in our full sandbox, the insertion of PermissionSetAssignments in our full sandbox has been significantly slower than in our production Winter 19 instance.  The pilot is not active in our full sandbox yet, but this starting happening immediately with Spring 19 (Some tests start failing due to time)

I put together a simple test class to test and these are the performance differences:

Production - Winter 19

15:17:16.822 (8827629591)|USER_DEBUG|[29]|DEBUG|Elapsed Seconds: 7

15:17:16.822 (8827669424)|USER_DEBUG|[30]|DEBUG|     User Count: 200

15:17:16.822 (8827754859)|USER_DEBUG|[31]|DEBUG|  Time per Perm: 0.035

Full Sandbox - Spring 19

15:09:16.105 (280139137861)|USER_DEBUG|[29]|DEBUG|Elapsed Seconds: 279

15:09:16.105 (280139252953)|USER_DEBUG|[30]|DEBUG|     User Count: 200

15:09:16.105 (280139479190)|USER_DEBUG|[31]|DEBUG|  Time per Perm: 1.395

So in the Winter 19 production instance, 200 PermissionSetAssignment records were inserted in 7 seconds.  In the Spring 19 full sandbox, inserting 200 records took 279 seconds.

@Sharon Liao If you don't believe this is related to the pilot, I will log a support case.