First off, sorry if I'm posting this in the wrong area.
My issue is I don't know if its lag, or an FPS issue when I run into massive mob groups (the game likes to stutter so I'm assuming FPS issue). I have a AMD FX-8350 (8-core, 4.1ghz) processor, GTX 970 graphics card, and 8gb of DDR4 ram. What can I do other than lowering video settings? Thanks!
It's usually lag. If that happens, check your latency (the small bar next to your inventor button). Lag has been and still is a huge issue in Diablo 3 when encountering huge packs of mobs. There are some ways to work around it (avoid big pulls when fighting a Health Link pack, turn off damage numbers, get rid of area damage if you're playing support)... but it's never gonna go away entirely.
Let's hope it gets better in 2.4.1.
Lag has nothing to do with your PC, your specs should be more than enough to run Diablo fluently, and the number of mobs encountered has nothing to do with your computer as all calculation is done on Blizzard's servers anyways.
I've always suspected it was their servers and not our comp's fault. They could alleviate the lag issue by using dedicated servers that only house D3's code and calculations from that code, but that might not solve the issue completely. WoW also has this issue, as some servers do double duty, thus creating some lag when the servers get full or are overtaxed. The latter doesn't happen very often, but often enough for network savvy people to notice.