It's hard to say what's taking so long.
Any LCD commands take the time set by LCD_COMMANDUS.
Each character takes LCD_DATAUS.
If they are set really high, it could take a long time on a 4-line display.

But whatever they are set at, this might help with keeping USB serviced...

USB_ASM_Service.pbp
http://www.picbasic.co.uk/forum/show...06&postcount=6