Somewhere in options, I think in performance, there is an auto-save feature. If that is set wrong, it could be that you are having the infamous "lag spike of death". If not, then you are being "minecrafted" (that's what I call it), and mc may be unplayable. My old laptop got so bad, it took 5 minutes to load. Please reply, and I hope this helps. I'm not an expert, I'm just trying to help out as best as I can.