Overlord,
Unless you set it up so that admins can quick edit everyones posts this hack should have no real effect on load times. If you install it as intended the only extra things that must load in showthread is the little icon and the hidden text which is only loaded in posts that belong to the person who is viewing the thread (a small avatar would take longer to load than the text).
Quick Edit uses the same standard javascript functions which vbulletin uses to make all of the little drop down boxes (like when you click on someones username, search, thread tools, etc.) so no extra javascript is loaded. You do have to wait until the whole thread finishes loading before quick edit becomes functional, but this doesn't add to the load times.
pco,
Thats a nice addition. I'll add a link to your post in the add-ons section in the first post of this thread so that others can use it.