Your card reader sets the offload speed
When you copy photos off a memory card, the card and its reader are usually the bottleneck, not the SSD. A UHS-II SD card reads at up to about 300 MB/s, so a full 64GB card takes around three to four minutes, and UHS-I cards are several times slower. Any portable SSD rated around 1,000 MB/s is fast enough for SD offloads; faster CFexpress cards are where a quicker SSD starts to matter.
Built for the camera bag
For field work, durability matters as much as speed. The SanDisk Extreme has a rubberized shell with an IP65 dust and water resistance rating, and it is one of the best-reviewed portable SSDs on Amazon. Keep a drive that small in the camera bag and you can back up every card the same day.
Keep two copies before you format a card
Photographers follow the 3-2-1 backup rule: three copies of important files, on two different types of storage, with one kept offsite. On a trip, the practical version is to copy each card to a portable SSD and keep the card until a second copy exists, on a laptop, a second drive or cloud storage. At home, a large hard drive or NAS holds the full archive.