You're only as safe as the dodgy websites you may or may not visit.
I use Avast AV, SpywareBlaster, Windows Defender and Windows firewall.
I don't visit dodgy sites and have remained uninfected for more years than I can remember.
The fact that your PC is "now very slow" is probably nothing to do with the security software you have loaded. Do you regularly carry out PC housekeeping with something like CCleaner?