Most decent-quality USB 3 flash drives will approach hard drive levels of performance, which may or may not be acceptable to you. Do be careful about where you buy from, because Amazon and eBay are full of counterfeits.
The better (but more expensive) option is to go with an external SSD. The Samsung T5 500GB is available for under £70 and has all the performance and durability you'd expect from a Samsung SSD.
If you've got an older laptop with limited storage, I'd recommend just installing a decent-sized SSD. It's not all that difficult, SATA SSDs are cheap as chips these days and you'll see a huge increase in responsiveness if you're still booting from a hard drive.
