There's a lot of large, cheap and SLOW sticks out there, as they try to get rid of older, slower NAND chips. That's OK if you're just doing the occasional backup or moving a few documents, but totally useless if you regularly copy 10GB+ and haven't got 5 hours spare.
If there's no quoted speeds, they're almost certainly poor - maybe 5-10MB/s read, 2-5MB/s write (or worse). 20-30 read, 10-15 write is acceptable for general use, but you can pay a lot more for "super-speeds" of 100MB/s+ - worth it if you often move/store large amounts of data.
NB Some cheaper USB 3.0 sticks are SLOWER than decent USB 2.0 ones, particularly using USB 2.0 ports, so don't be fooled by the hype - read the smallprint (or use Google). And pay a bit extra for a known make with a 3+ year warranty, as cheap = poor NAND = low endurance (less erase cycles before failure), and could take your data with it when it dies.
