1) How it started.
Our most important solution has about 60 database files, some having up to 14 tables, some only one. We have relogin scripts in each and every database file and some centralised "relogin all" scripts that calls them, allowing us to enter in all databases at the same privilege level.
Since FMP 15 we noticed a really long time (1-2 minutes) in the execution of this "relogin all xxx" script, while in FMP 13 it was over in 4-5 seconds.
2) Going deeper.
On my development iMac (i7, SSD, 16 GB RAM, running the latest El Capitan) I noticed a huge difference running the enclosed test: it took 0.99 seconds to run on FMP 13 and 64 seconds on FMP 15, for 100 Re-Logins.
FMP 15 also had the solution opened (residing on our dev server). FMP 13 had no other DB's open.
At 3 different clients, running the latest Sierra and FMP 15.0.3, a very similar iMac displayed the same problems.
Useless to mention, we use this script a lot when assisting our clients.
at home, on my MacBook Pro i7 retina 8 GB, SSD, the results from FMP 13 and FMP 15 were similar - around 10 seconds for 1000 Re-Logins.
Any clue ?