A million different things could be causing it
If it's a network issue (mouse etc., can move fine, just not getting movement/actions happening ingame as quickly as they should) then the first place I'd look is your router, make sure the firmware is up to date and nobody else is doing any high-bandwith things at the same time, second place I'd look is the connection between your computer and the router. If cable, try replacing them with new cables. If wifi, check if there's other wifi signals interfering, there are free apps for apple/android that can do that, just search the app store.
If it's a local issue (everything is locking up) then the first thing I'd be suspect of is your hard drive. First up check to see if anything is maxing out your read/write capability as a priority, giving Mir less priority (the game does a lot more disk reads than you would expect, so it needs that access!), windows update and virus scanners are notorious for that, I personally disable them while playing. If that all looks fine, then find some disk health scanner utility to check if your drive is behaving correctly.
There's obviously a million other things it COULD be, but these are the first (most likely) areas I'd look at
