I'm suffering what seems to be a common ailment... Performance starts to really lag about 20-30 minutes into a fresh launch and file.
I've been researching this issue down to the nitty gritty for a while now, so I've plowed through the most common suggestion for sure.
==> I'm working with large and complex files utilizing a very wide range of illustrator effects, masks, plugins, distortions, 3d, blend modes... all at once, often at huge artboards! Multiple artboards! Embeded images! You name it, I'm doing it.
I've already had to significantly reel in my workflow and limit my filters (no gaussian blurs... no svg filters like drop shadow etc.) but the thing is, I really need to work in this way because I work with constant revisions on everything and the ability to change any piece on the fly is becoming both the glory and the horror that is illustrator.
I built my own custom Windows 10 machine last year, tailored to graphic design. I didn't buy the top end components at the time since I didn't have the money, but I've since upgraded a number of components and I'm hitting the ceiling in some areas. To buy stronger components in some areas I pretty much need to start at motherboard. sigh.
===> Windows 10 x64
32gb 2133 ddr4
X2 512 gb samsung SSD's (all my working files are on my boot SSD
Radeon R9 380 w/ 8gb (upgraded after crap performance from a GTX 970 2gb)
It's a shame because illustrator is an amazing tool. Combined with the set of plugins offered from Astute graphics, I have a very similar toolset to what photoshop can do. But in vector, it's very slow.