Graeme,
thanks for calling attention on this. I did a couple of quick tests, but so far I wasn't able to replicate the slowness with Directory Opus.
Our test process involves only the built in file managers on Windows and OS X and so far we haven't experienced any speed issues with our Shell Extension.
For now I can just agree with their support that disabling our Shell Extension is the quickest way to address the speed issue. I posted a report for our engineering to check this.
Marton
Marton Kiss
VP, Product Success
GRAPHISOFT