Brush painting slow

Mac OS High Sierra 10.13

Adobe Photoshop CC 19.0

Brush set to -

Mode - Normal

Opacity - 100%

Flow - 100%

Smoothing - 100%

Smoothing options - nothing ticked

My colleague's brush paints very slow and won't keep up whilst painting, it's causing issues (obviously) whilst editing.

I checked brush settings (in PS preffs) and mouse settings (in System preffs), his are all the same as mine (my mac works fine.)

I can't work out what the issue is, nor can he. Anyone any ideas?

Alex