[Complete] Unscheduled Minor Patch - April 11, 2014

When the Evo World server is heavily loaded, during 2x events and some days on peak hours, the spawn time is 10 seconds.
When the load is light, it's 4 seconds.
It might be that the first few instanced maps have a 4 second spawn timer, but the rest have the 10 second timer.

I don't know why after a maintenance it seems to always start up with the 10-second spawn (and ignores cores?)

This is true, but you can also get 'instant' spawn. I have had this happen on maps outside of EVO world as well. 'Instant spawn' is a condition where a replacement mob pops up the second you kill one. Ine EVO, it's easy to tell what you get if you can clear the map on link 3.
1) 10 second spawn (super crap spawn) - You'll clear the map and then twiddle your thumbs while you wait for the spawn to happen
2) 4 second spawn - You'll clear the map and then see all the mobs spawn at once after a short delay.
3) Instant spawn (the one you WANT and are SUPPOSED to get) - You'll kill along nicely and get a constant stream of ongoing spawn.

At first I thought 4 second spawn was the fast spawn, but it turns out that Instant spawn is the true EVO spawn. I have had all 3 happen to me on the same day on consecutive runs. Unfortunately, whatever spawn type you get will remain throughout the run. When I get 10 second spawn, I give up and leave. Speculation has been that different channels get the different spawn rates. I think it's more random than that.

EDIT - If your kill rate is slower than usual (due to HP modifiers or what have you), then it;s easy to mistake 4 second spawn for instant spawn. BUt, instant spawn is real and if you 1hko then instant spawn is the ultimate in training. I had it most of the time while I was training my 5 beast tamers to 130 and it was just awesome.