The issue is caused by the file that is supposed to make the prediction of your autocomplete better. Go to $UserBaseDirectory/.Mathematica/SystemFiles/FrontEnd/SystemResources/FunctionalFrequency and check the .m-file there. Everytime this happens to me, this file was several hundred MB large. Deleting it solves the problem.

The files in this directory store your autocompletion history to help the system make accurate completions based on your past choices. Several hundred megabytes seems quite excessive. If anyone here encounters this problem again and there is a very large file in this directory, you can help me track down the problem by sending the file into tech support along with a description of the problem (assuming that deleting the file actually does cure the issue). I'll get notified and be better able to fix the issue.

Tom

Thanks for the tip David. I had thought of delving into the preferences to see whether I could turn autocompletion off, but it's a feature I rather like! In any case, after multiple crashes, Mathematica 10's kernel decided it had licensing 'issues' and I think had become corrupted. I had to uninstall it. I hope a speedy bugfix for Windows users is forthcoming, but for now, I'm back to version 9.1.

I keep getting the same issue as the OP. I've tried all of the above and it still freezes upon auto-completing even after I've increased the time and write simple functions as Plot, Solve, Factor etc. which takes less time to type than the auto-completing threshold I've entered.

I solved the freezing problem! Deinstall the Graphik driver for NVIDIA GEFORCE 9400GT. Mathematica 10 starts without delay.

No problem, it happens quite frequently to me. A shame as well, since autocompletion without this file favors some weird suggestions sometimes (like when you type Integ... it will suggest "Integer" rather than "Integrate").

Tom: I just reported a bug of this kind and sent the file (407mb) to tech support.