midi keyboard input very laggy (slow response)

I'm running MS on MacOS High Sierra, with a USB midi keyboard attached. Sometimes it works fine, but more often than not, the response to keyboard input is very slow - like 1 or 2 seconds before a note appears on screen.

This matters when you change the note value using the Mac keyboard (type 5 4 6 etc) as this change is picked up immediately, and affects notes you have already entered, if they have not yet appeared on the screen. So you have to wait for each note to appear before changing note value, which means taking your eye off the printed music you are copying from (in my case).

Because of the intermittent nature of the problem (some sessions it does not happen) I think it is software related, but not necessarily anything to do with MS. The computer is used for many other things.

Does anyone know how to improve this, by giving the keyboard top processing priority?

I've tried restarting MS and the keyboard. I think restarting the Mac will help but that's a very slow process!

Thanks in advance