If you use Sophos anti-virus on your Mac, you might have noticed some issues when you ran Time Machine alongside. The problem is that Time Machine backs up a lot of files. Sometimes, it backs up thousands or even hundreds of thousands of files. Now, Sophos sometimes erroneously checks all these individual actions to see if they are a threat to your Mac.
As you can imagine, Time Machine can get really choked if it has to wait for clearance from Sophos all the time. So, the only workaround for this is to quit Sophos temporarily when you run Time Machine. Also, you can try downloading the latest version of Sophos to see if they have released a patch to fix this Time Machine issue.
Most users have said that Sophos only causes problems in the latest Mac operating system in Mac OS Sierra. El Capitan, Yosemite and Mavericks users have not complained of this issue. It could just be that Sophos didn’t release an optimized version of their anti-virus for Mac OS Sierra specifically. But, latest press releases suggest that Sophos has released an update after the Sierra launch, fixing this exact problem.
But if you still have issues after downloading the latest version of Sophos, it could mean that you have other problems that need to be fixed on your Mac. For example, there might be rogue apps that are eating away too much CPU and RAM resource when you run them. They could have gone rogue because of an incomplete installation or because of corruption that might have been brought about by you deleting some app files, instead of maybe uninstalling it, like when you might have tried to uninstall an older version and install a new version.
In any which way, a Mac that has suddenly slowed down can almost always be brought back to normal with the right steps.