USB is the bottleneck with external. The external disk itself (likely SATA) should be faster than its USB connection. So getting a faster disk for your external drive won't make your USB accessing faster.
Internal should be faster than external, and solid state internal should give you the best performance but with silly pricing.
Check the specs for the hard disk controller inside your computer. It will ultimately determine how fast of a disk you can use internally.
I use a second internal hard disk to store files and sync it with an external USB drive for a backup. Thus all my accesses for editing purposes are from/to an internal disk.
For desktops you can get removable drive bay enclosures that allow you to hot swap an internal hard disk without disassembling your computer case, and it operates at internal disk speeds. Some laptops like my Lenovo ThinkPad also have such a device. They are very convenient.
Ones for desktops come in two parts. One part (the enclosure) is installed into an empty 5-1/4 inch drive bay and connects directly to your hard disk controller. The second part (the drawer) contains a hard disk and slides into it the first. You can usually buy the parts separately, so you can have several hard disks - any one of which you can slide into the enclosure and use just as if it were another internal drive.