The performance of search will depend on a few things
Size of target being crawled, bigger it is the longer it'll take, naturally. Also, the performance of the target, if pages are loading slowly, search will run slower.
Your farm infrastructure: services per server and server performance. How is your farm setup? Is search running on its own server or shared with other services? How many users are actively on your sites? What's the hardware specs of the servers running search? These are some of the considerations for performance of search.
I'd recommend running a full crawl when user traffic is lowest, and server resources are most available, usually late night, early morning, not during a system backup.