Yep, a SATA SSD that runs over USB 3.0 is very fast on the RPi4, as long as you make sure "UASP mode" is enabled (this can take bit of tinkering). You don't even need external power, just a $5 USB to SATA cable.
Anecdata: I’ve found USB powered external drives to be unreliable on the Pi, even when powering the Pi with an official power adapter. I’d recommend using an external drive that comes with its own power adapter.
Counter anecdata: I've got a Ceph cluster running on multiple RPi 4s using external Seagate 4 and 5 TB portable hard drives. So far zero issues for about 1 year since I bootstrapped this cluster..
I use a combination of official RPi 4 power supplies and an Anker 6-port USB power block that I'm pretty sure tops out at less than 3 amps.
More importantly, everything is plugged into APC UPSes.
Generally much faster than the traditional protocol[1].
However some USB SATA IC's have issued with it on Raspnberry Pi[2], requiring one to disable UASP. I have some JMicron-based USB-SATA adapters which falls into this category.
What are you using for storage? Powered HDD over USB or something?