Not saying you should move at a crawl with an MG but I held a few roles in the infantry and I moved noticeably slower than dudes just running around with an M4.
Far as collision models go. Clearing a house with an SPR or SAW is definitely harder to do than with a MK18 CQBR. You have to index the weapon over your shoulder (Exaggerated high ready) or under your arm (high port) to clear doorways and stuff like that.