I want to run Raspbian from a USB flash drive on a Raspberry Pi 3B+.
What would be the performance difference in doing so, as compared to running Raspbian from an SD card?
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You are using a Raspberry Pi 3B+. This model supports booting from an USB storage device instead of a SD Card out of the box. Just flash the USB flash drive as you do with a SD Card, put it into an USB port and be sure you do not have an SD Card inserted. Otherwise it would boot first from the SD Card. For details look at USB mass storage boot.
The Raspberry Pi 3B + supports USB 2.0 specification which allows for transfer speeds up to 480Mbit/s or 60MByte/s.
USB flash drives can reach up 33MBytes/s, but there seem to be some limitations in the NAND itself so it will come down to the type of stick you get. However, the sd-card bus speed can reach up to 20MB/s read on the raspberry pi. So, the better choice here is a high-class USB 2.0 USB flash drive.
On the other hand, the payment would be concerned. It cost much less to buy USB 2.0 USB flash drive instead of a UHS-3/Class 10 sd-card.
In conclusion, I see a comparatively small benefit in booting from a USB flash stick, the price, and the speed.