There is two ways to do this.  

Either BLOCK the 720 ones with a filter rule like

-*720*


OR the better way is to block EVERYTHING and then allow only what you want
(and what you need)

-*
+(A set of rules for the Pages to get; that have the video links)
+THE_site/VID_Dir_That_exists/*1080*




As for the slow, You should never beat a sever into the ground by pulling
100MB, All those limiters that you most likely removed or set to very high
values let you hammer the site, SO the SITE is cutting you back to slow most
likly.

HTTrak is just as fast as your browser (or faster) in retreaving files.  It
Also has to process (crawl) each one to discover links, and that can take time
depending on your PC