What are most common issues which slow down the computer speed while anti-virus is scanning?

Because  antivirus are scanned full sysytem with C: Drive like OS Drive. when this drive are scanned that time your windows and system folder also scanned thats the reason your computer slow down when anti virus scanning and aslo depend on Hard drive read write speed.

use PerfMon icluding all disk counters , CPU , DISK queue length and check what is causing the issue. 

https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiY24KPtszXAhVBpo8KHbLjB0QQFggnMAA&url=https%3A%2F%2Fsocial.technet.microsoft.com%2FForums%2Fwindows%2Fen-US%2F4a782e40-bbd8-40b7-869d-68e3dfd1a5b4%2Fwindows-update-scan-high-memory-usage%3Fforum%3Dw7itproperf&usg=AOvVaw2NBbfjgibvNEYNKIkgnoQA

the antivirus itself could slow down computer processes till it is done scanning

if you run a full solution AV, then it can use up most of your PC resources:

i agree with mr. aemand pers

Antivirus itself is the first reason why a pc is slow during a scan:

Which Anti Virus do you run? and which version of it (premium, free, solution suite)? 

if you run a full solution AV, then it can use up most of your PC resources:

while AV is running, open your task manager, look at the performance tab and check CPU and RAM usage. Check processes and sort out by memory usage, you ll see which prog is using the most, more likely your antivirus.

if the above is correct, schedule a scan while you re not using your pc.

spyware & malware are active that why slow  ir You have a virus in boot sector /

hard drive is full / bed sectors  in hard drive