I'm backing up my 256 GB Pixel 8 on a very fast 256 GB Kingston DataTraveler (which I can recommend, fast and cheap).
For around 120 GiB for the initial backup, it took less than an hour.
Many USB flash drives are super slow, a portable SSD or a fast flash drive is really helpful.
There's a large number of very fast portable SSDs and also very fast thumb drives on the market.