With a 250GB Sandisk Extreme external SSD read performance with FreeBSD is lower than Linux and macOS. I have tested this on FreeBSD on a Thinkpad x270 and 10th Generation Intel NUC. On Linux on the Intel NUC and from MacOS on a 2018 Macbook pro.
I evaluated read and write performance by using dd (with only path adaptations for Linux and macOS) on the device:
dd if=/dev/zero of=/dev/da0 bs=1m count=50000
dd if=/dev/da0 of=/dev/null bs=1m count=50000
On all platforms write speed is approximately 200MB/s
On Linux and macOS read speed is approximately 500MB/s, while on FreeBSD it is approx 200MB/s
You should be aware that Linux buffers reads and writes to/from the raw block device, /dev/xxx . FreeBSD does not.