Short answer: It sounds like the the solution is to delete the file CodeAssistHistory10.4.m See below on how to find this file on your system.
On my (Linux) system the file is in a directory .$HOME/.Mathematica/SystemFiles/FrontEnd/SystemResources/FunctionalFrequency, if that helps you locate the file on your system. Sometimes this file on my system suddenly grows to 100's of MB and the front end slows down or stops entirely after typing only one or two characters. I originally found the solution here on the StackExchange.
There is also a work around for this problem. Simply turning off the autocompletion feature of the MMA front end will prevent MMA from slowing/stopping after one or two characters are typed. From the toolbar, select Edit->Preferences. On the Interface tab, uncheck the "Enable autocompletion with ..." option. That should allow you to use MMA without the odd behavior.
The following MMA commands should show you exactly where the CodeAssistHistory.10.4.m file is on your system and how large it is
dir = FileNameJoin[{$UserBaseDirectory, "SystemFiles", "FrontEnd",
"SystemResources", "FunctionalFrequency"}]
SetDirectory[dir]
FileNames[]
FileByteCount["CodeAssistHistory10.4.m"]
If the above code says the CodeAssistHistory file size is only a few kilobytes, then that is not the problem. But if the file size is megabytes or larger, you should delete the file.
Don't forget to turn the autocompletion back on, if you want to use it.
