For a virus checker to do its job, it has to check the file before the file
is used to make sure it is safe. This takes time, not a lot of time in most
cases, but it does take time. This will cause ALL things done with the
computer to slow down. The question is "to what extent?" That will depend on
the antivirus program and its settings. I suggest you contact the maker of
the antivirus program for help with the settings.