My guess is that you should be capturing on BRIDGE interface. CPU usage and memory usage seem not to be the bottleneck. Try to switch capturing the BRIDGE interface only and make sure you capture only one way, e.g use source address of your local lan.
I switched to bridge as per your suggestion – still no improvement. Web pages continue to load very slowly when HAVP is activated – logging is turn off(only logs webpages with virus), image scanning is also off. Is there a way for me to temporarily turn off the antivirus and leave the http proxy on?