The scroll list is only refreshed 4 times per second (this is adjustable from 1 to 10 in the "Options" tab of the radar), but it does take some time to render. This said, you can close the radar once you got the avatar(s) marked: just make sure it got the "Announce incoming avatars even when closed" box checked in the Options tab, so that even when its floater is closed the radar keeps working in the background.
Also, "expensive" via the radar for a (specific and occasional) purpose such as yours is better than expensive in the mini-map for everyone else not caring about such a feature.. Bloat is the enemy.
Lua is your friend and OnRadar() will allow you to distinguish important alerts with any mean of your choice (sound, notification tip, additional chat, etc): the 'marked' parameter could be used for such a purpose together with 'range'...
I might extend the Lua/radar interface in the future.