Error -43 is a generic error that indicates some problem with accessing files.
If you are copying a very large number of files at one time, the Finder doesn't always handle that well, and you may get the dreaded "error code -43". In those cases, using a backup utility like Carbon Copy Cloner to do the file copy may succeed where the Finder fails. This was an issue in the past; I don't know if it's still present in current versions of macOS.
In this case, if everything is slow, that suggests to me that you have a failing hard drive. Once a hard drive begins to fail, you may see increasing slowness – and as the failure gets worse, your Mac may hang for long times while doing innocuous things.
What does the Mac say in System Report / System Information about the S.M.A.R.T. status of the drive?