Why is emptying the trash so much slower than rm? If there are a lot of files in the trash, sometimes the Finder will display a progress bar saying (for example) "Preparing to delete 62,736 files", then another progress bar for the actual deletion. By comparison, deleting those same files from the command line is nearly instantaneous. 

What is the Finder doing that takes so long? Is it detrimental to my system to skip that by deleting files via rm?
This is a case of "destructive" vs "non-destructive" editing of files.
Like in old days of cutting and gluing together audio tape or film, editing then was destructive like RM command is. You type in the command, hit ENTER and RM is immediately executed.

On the other hand, moving files to the TRASH folder is just that, and the system responds, "wait while I figure out how much stuff I need to move". You pay in time for the ability to UNDO a possible accidental erase. In the old days we were usually not so lucky in computers or with film or tape. :-)