Such apps can "scan on open" and more so you pay for the security with lower speeds.
Also a new install of most antivirus will have it scan all drives (hours, days or longer) so this will drastically slow the PC for a long time. Nothing wrong about that but few tell you about these issues.
Go back to how your machine worked well. Here I find Windows Defender to be enough.
Running Windows 10 64 bit (1803) all the latest update HP PC with I7 chip (3.40 mgz) and 8 GB . All was good until I installed Bitdefender 2019 . Now ALL Apps open slowly but once open they seem to do fine. No Internet issues other than Browser opening slowly (Firefox Quantum). Time to open any app from a shortcut is sooooo very frustratingly long it has taken most of the joy out of computing. I have run SFC and Chkdsk and a couple others. One scan that was supposed to scan AND repair got stuck at 12% for hours so something happened there. Have a ticket into Bitdefender but thought I would try running this by the forum. Any help would be much appreciated.

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