Extreme slow Web filter performance

Hi,

I am current running 6.4.4 with 60F in profile-based mode. the policy and the web filter profile are both in Flow mode. And I only use Certificate-inspection. 

I have noticed, If I enabled any "Fortiguard Category" in the web filter profile. then when I test with speedtest etc. I get 70% performance hit every time. I can understand 30% hit of AV profile since it need look into the the contents even it's not decrypted. But I assume Web filter with certificate-inspection, only look into CN in the handshake and of course none SSL traffic. Looks like the speedtest is none SSL traffic. Is those kind of performance impact normal? If I run web filter in monitoring only mode. the performance hit is almost Zero. Thanks.