About less action..
I've not tested it yet, but I think there is a command in DFHack:
force siege EVIL
So if you have the population/weath requirements, and an evil goblin civilization exists in that world, that will force an attack. Not as good as being surprised.. But I suppose you can decide what chance a siege will happen yourself and roll a die or flip a coin each season.

I've had good luck with Undead Sieges (maybe because they don't wander the map and conquest?) but Undead are brokenly overpowered at the moment so it's not a substitute.
A smaller world does seem to help, but it doesn't matter what kind of PC you have, at times the game can get slow. I recently upgraded to a 4Ghz Core i7 that turbos up to like 4.5Ghz, 16GB of the fastest RAM with the lowest latency I could find and while it did make a nice improvement it didn't make as much difference as it does for other games. I had a savegame that was 22FPS on my older PC (huge undead siege) while my new system doubled to 44fps. Much better but still a bit slow, and that was a fairly young fort. World Generation, however, is WAY faster. Many times faster.
Best thing I can suggest is read about how to design your fortress to maximize performance, that seems to help more than anything really.