everytime i go to play on 1.8 pre-2 (1.7 is boring) I get massive lag spikes, no matter what settings i have, no matter what resource pack i use, i always get a massive lag spike. i gave minecraft more RAM, i always set it to high priority under the task manager, i go from (at the lowest possible settings) 104 fps to 32 fps every 5 seconds, on higher settings that i use normaly, i go from a stable 60 fps, to 3 fps every 5 seconds. that is using the default texture pack.
Something isn't right with it for some users. I'm still not sure what is going on. If I stand still it loads pretty dang fast, but if I do anything apart from standing still, it all goes down the drain. It became pretty much unplayable for me. I'm not sure what's up with that, but not everyone is having this problem, so I suspect the performance update just isn't keen for certain types of computer architecture. It is still a pre-release after all, so it may get fixed. I'd say give it some time then post again if it's still having trouble when it hits an official release.
1.8 pre-2 and pre-3 lag really bad for me for some reason. Pre-1 runs pretty smooth so I'm staying with that for a while. Hopefully the final 1.8 fixes that issue.
i was having the same problem when playing the snapshots, and it still seems to be happening even after the official update came out. every couple of seconds i get really bad lags spikes. i usually get about 90 frames but this update is just really laggy for me for some reason.
Open minecraft, click edit profile, check JVM Arguments and type: -Xmx500M -Xms500M, if this is not working try changing it to -Xmx1000M -Xms1000M
to fix it you need to go into video settings and turn off vsync. hope that helped