I have barely any space left on my hard drive so I want to buy an external hard drive but I was wondering if it will effect my FPS or just the loading time.And how long will a game like GTA 5 would take to load on an 3.0 USB port?
USB 3 is pretty fast; 480 Mb/s. That's megaBITs, not megaBYTES. It's roughly 60MB/s.
Comparitively, your SATA II or III connections for an internal hard disk communicate at up to 600GB/s, which is around 600,000 MB/s.
Most traditional hard disks can hit up to 128MB/s read speed, where an SSD can hit 6 times that. This means that if you use a USB hard disk, you're looking at a limit set by the interface, and you'll get 60MB/s.If you use USB 3.1, you'll be limited by the hard disk read speed of 128MB/s, so you'd get identical performance to an HDD connected via SATA.
With an SSD, you're still interface-limited. If you use USB 3.1, you can get up to 1.25GB/s and will see this in your drive performance. Note that this speed is faster than an SSD read speed, so again, you'll get normal performance for an SSD. If you plug an SSD into a USB 3.0 port or earlier, you lose the speed benefits of the SSD entirely.
What speed is your current hard disk? At data loads? Is the disk you are getting faster? Is the UBS port transfer rate faster?