USB 3.0 ports maxing out at 40 MB/s

Try an SMC reset and PRAM reset (hold the PRAM reset for at least three chimes so it activates several times).

Did you try both USB ports? FYI, the right USB port is the one mounted directly on the Logic Board on a MBAir.

Maybe you have an older or even a defective USB cable. Are the USB ports clean and undamaged?

Maybe the external USB SATA controller for your adapter/dock/enclosure is not working correctly or is not compatible.

Disconnect all other external devices while testing.

Are you using a USB hub? Try connecting the drive directly to the laptop.

I've personally had issues using USB3 drives connected directly to the Mac's USB ports. I've found a good USB3 powered hub with UASP support helps by providing extra power to the devices if needed and it also helps to buffer devices from the main system.

FYI, a hard drive may max out at 40MB/s so it is not a good item to test.

Believe it or not, but some low end budget SSDs (even from a name brand) may only perform the same as a traditional hard drive.

Is the drive being tested being indexed by Spotlight? Or scanned by AV or other security software? Is the external drive using any proprietary driver/software to use/access it?

Try testing the speed while in Safe Mode which will load only the minimal drivers and keep Startup and Login items from launching. It is normal for the screen to flash or flicker in Safe Mode due to the very basic video driver being used.