Jerky and slow when inserting blocks (Version 6.6 Standard)

#1
We have 4 identical PCs (Windows 7, 12 GB RAM) all with basically the same software. On three of them, inserting blocks is an absolute nightmare, because the cursor becomes jerky and unresponsive. But on the fourth computer, this does not happen!

It appears to us that the problem is due to entity snapping - on the one that does not become jerky, the blocks do not try and align to other objects whereas on the other three, they do.

However, we cannot work out what is different between the PCs. Is there a user setting that controls this behaviour?

Any help greatly appreciated!
cron