Why does restore take a long time?
The backup size is simply used 8k pages. These 8k pages are part of the MDF. On restore, the MDF and LDF files may need recreated (if already existing and different in some way) or just created

So backup file size isn't an indicator of space used on disk by the restored database. And this ignores backup compression too

So, if the MDF is 100GB (with 2GB actually used), : unless you have Instant File Initialisation on, then the 100GB needs zeroed. Note the LDF must be zeroed.