I have just upgraded my Mac Mini to El Capitan from Yosemite.
I then upgraded FileMaker Server from v14 to v15.
Using FileMaker Pro & ProAdv & Go, my hosted files are available without excessive time delays.
Files which were previously used with WebDirect now take an exceptionally long time to load.
The spinning worm/circle is there for minutes. Then the layout is built piece by piece, very slowly.
The WebDirect Launching page also takes an age to load.
(The Admin Console is also very slow to appear)
I closed all my hosted files on the Server, leaving only FMServer_Sample available.
This demonstrates the problem as well.
Sometime the attached error message appears (not able to load some Widgets).
Even if I use the LAN only for my WebDirect connection (specifying the IP Address of the Server), I have the same slowness.
I have tried Safari & Chrome & different machines & locations.
have a look at (be prepared to wait a bit)