While all antivirus programs slow down a PC a bit, some are definitely better than others.
Bitdefender, Norton, and Kaspersky (and few others) have all been reviewed to have the least impact on system performance compared to some of their counterparts. Even using Windows Defender can slow down the PC a bit.
https://www.av-test.org/en/antivirus/home-windows/
I've only used Bitdefender's free antivirus program on a few PCs, and I didn't notice really any difference (compared to the PCs with Norton on them).