I've got an 8MB XML file, editing this on Win XP. The typical line in the XML file is about 8000 chars long.
With word-wrap ON, I can fly through the file, keyboard edits are pretty fast.
With word-wrap OFF, ST slows to a crawl. Responding to a key event slows to about 1 per second. Unusable.
Please look into this! At least the last month of dev builds has this issue. Changing the highlighter from XML to plain text makes no difference.