What you can do is disable the GPO (as you have done already) and you can run a gpupdate /force remotely on machines that you "know" have this slowness issue. When you disable the GPO it will not automatically undo the changes. All GPO updates are cycled anywhere from 60-90 minutes. Doing a gpupdate /force does not always work the first time either.
Another thing you can do us us the MMC and add RSOP as a snap-in and this will allow you to check the computers to see if they still have the policy applied to them. Another way of doing this is from ADUC.
To do this from ADUC, find the computer in question, right click and go to "all tasks" and select Resaltant Set of Policy. This way you can check this remotely rather than going to the machine.