Any stealth virus have be a system process, that is why antivirus check for boot entries and modification by other process. This makes system startup delay.
If you observe once antivirus is loaded while startup, system start functioning as normal speed.
2nd, thing make your Antivirus slowdown system is real protection. When you download a file it checks the downloaded file soon it finishes, you can see the delay between 100% completion and completing the process of download.
3rd, Whenever you open a directory it checks for changes and auto execute files. This one also you can observe when you have a virus in your file system. Antivirus normally doesn't do anything but soon you open the folder it quarantine the virus.
More number of installed programs and start up, slow the start up, simple equation.
I have 8 GB of RAM but still my start is very slow and system is kind of freezed up till the antivirus loaded. I tried every tweak but no solution. Finally I just hibernate the system every time.😀
If you don't have hibernate option in menu just run
Ctrl+r --> shutdown -h