Logic Pro X is sluggish when using external audio interface

I'm using Logic Pro X 10.6.2 and a problem has started that I've never had before. When I try and use it with my Focusrite Clarrett2Pre, it is sluggish and unusable. I cannot use the controls without it loading for a long time and the meters jump up and down. But if I use it with the built in audio, it runs smoothly.

I've tried reinstalling Logic but it had no effect. I've also reset my preferences and still nothing. Has anyone else had this problem who has a fix?