Seeing the same issue with windows Server 2008 Datacenter SP1.
Well disappointing to hear it is also on the Windows server side too. I wonder what changed. I'm hoping it will be a bug fix that FileMaker takes care of and it goes back to being faster like in previous versions. <fingers crossed>
I am using gfxCardStatus on the Mac and observed that FMP15 mostly running utilizing the internal graphics card where FMP14 switches to discreet graphics card on the MacBook Pro more frequent.
Not sure if the graphics handling is somehow related.
I had a similar issue when upgrading first server, and then users, from 12 to 14.
That were because change in ports (80, 443, 5003) used in uploading files to the server. After updating the firewalls (both software for LAN and hardware for WAN) it was solved.
I think it worth to check the documentation and verifying the firewall settings accordingly.
With best regards Magnus Fransson.