Freezing during input

I am getting a similar problem with D4 with a new - at this stage - three page project. This started happening after updating to D4. With 3.5 there were no lag issues.
I thought it might have been due to the size of a project I have just completed of 330 pages - not so.
I have checked for things like sampling rate of my Behringer U-Phoria UMC404 or potential incompatibilities with other sound activities. All set to 48Hz/16 bit.
I’ve done an optimization of the computer - deleting temporary files; prefetch files; checked for corrupt system files.
I’ve stopped the VST driver and restarted Dorico. I’ve restarted the computer.
I have heaps of RAM and Disk space on an HP i7 - 6 core; 16Gb RAM; 2Tb internal HD.
Any thoughts?