I found out that I can slow down my game time without slowing down other players in co-op (similar to using slow-motion) by lagging my PC (I shoot the akimbo grimms at a wall and the dust lags my crappy computer). Would it be allowed to use this technique in a speedrun?

Hey, we do not want to enforce a minimum frame rate rule and exclude low-end players from speedrunning Payday 2. However lagging purposefully your game using particles would be considered as a banned exploit because it can't be reproduced on all platforms and PC specs. Similarly, while down/mask-up slow-mo is allowed in glitchless runs, we've banned in the past an exploit that would put the host - and only the host - in a perpetual slow-motion state.