Anti-Virus is the second most likely Threat Prevention blade to cause performance and/or high CPU utilization issues after IPS. Anti-Bot is relatively low overhead and is probably not causing your issue.
Generally, you don't want traffic travelling between high-speed internal networks to be scanned with Anti-Virus unless you have a quite beefy firewall. There are two primary ways to ensure this does not happen, by creating a blade-based (not protection-based) exception or by adjusting the profile-based properties for the Anti-Virus blade which has many performance-impacting settings you should check. Here are the relevant pages from my Gateway Performance Optimization Course covering these topics which you should find helpful, the blade-based exception example shown is for IPS but will work for Anti-Virus too:
Gateway Performance Optimization R81.20 Course
now available at maxpowerfirewalls.com