Hello Stefan,
good idea. The problem you have is because I am not overwriting user preferences any more, and you still have word wrap always on for text files (i have reverted it to Off in new installations).
But any way, I think, even if user have it for small files enabled, he would not like this for bigger files.
So I would add xml flag controlling that (do not think that GUI flag necessary), with default for example 2048 kb.
I have done similar optimization already for parsing statistics. It is not done if file is bigger then 2 megs by default, because it used mostly for editing and not used in viewing. This was done to reduce memory consumption.
Disabling of syntax highligthing I dont think woud give some benefits. For txt files it is done only for visible part, so cost nothing. All the rest of functions are done in separated threads, so multi core CPUs should not influence too much.