Alright... You know, I'm still trying to figure out what you mean here... Please, write plain and explicit phrases (with a subject, a verb, and a complement) and give precise repro steps... I'm going to try and *guess* what you meant:
Setting yourself to "busy" doesn't slow down anything... However, if you let the viewer window loose the focus (which is likely to happen if setting yourself busy was done so that you could deal with other stuff in another program window) then the viewer enters its low frame rate mode (the viewer voluntarily slows down its refresh rate to alleviate the load on the CPU, so that you can do other stuff while staying logged in SL). This is not a bug, or (if you didn't loose the focus when the slow down happened) I cannot reproduce it.
Even if the list of nicknames is empty, the list of words to highlight should at the very least contain your avatar's first name (for newly registered avatars, it's the user name) and last name (for older residents only, since for new ones this name is "Resident" and not taken into account). So, the strange thing is that the log says that the list is empty. In fact, winding back to your first three words "login via link" I supposed you meant that the viewer was launched after you clicked on a slurl in your web browser... so, I did the same thing and indeed, doing so (which means using the --autologin command line option of the viewer) results in the FirstName and LastName settings *not* being set, resulting, when the nicknames list is also empty, in an empty list of words to look for, which in turn triggers the crash (since the list normally contains at least one word, the avatar first name, no check is done for an empty list).
I've not the tiniest clue of what you are speaking about here... But nevermind, since I spotted the bug and it will be fixed in the next release.