NetGuard is a bit of a puzzle. I think it goes one step beyond SiteAdvisor, which is mostly advisory, and actively blocks connections which are on a blacklist somewhere. That would involve a delay while it does online lookups of the various blacklists and searches them for the site you want to go to.
Those people who only go to a limited range of known and trusted websites can probably turn off Netguard to improve loading times, although of course if any of your usual websites should be poisoned by a malware injection then it leaves you wide open to the possibility of infection if the malware is using some exploit that McAfee hasn't been able to detect yet. It's a calculated risk. If someone's net browsing is of an adventurous nature - especially if it involves P2P, online games, and, um, adult sites - I would say leave Netguard on and accept the loss of browser performance. Otherwise, you can probably turn it off.
The best compromise might be to have more than one browser, and use one with Netguard for visiting sites you haven't been to before, or which might be a bit risky; use another without Netguard for visiting trusted sites.
As an experiment I turned Netguard off in Chrome, and loading of pages does seem to be slightly faster.
Edit - On XP, with IE8, loading is definitely faster. If I get bored sometime I could do a comparison of loading times with/without Netguard using Microsoft's Visual Round Trip Analyzer or IBM's IBM Page Detailer Basic, in the Help for which I find the following :
Often the content for a single page is built from items retrieved from multiple servers. These items may be from multiple servers at one site or multiple servers across multiple sites.
If NetGuard is checking all of these it certainly would slow things down, while undoubtedly making browsing safer.
It would help if there were some technical information for NetGuard but I haven't found anything yet.
Thanks for the response. My browser performance is night and day, so killing off Netguard for the IE8 process really helped. I left it on for everything else and so far no real problems. I have a limited list of sites I visit, and don't peruse the riskier ones anyhow. For now I'm satisfied this is the issue, but I would think McAfee might want to look into this further as I've found other threads indicating this process may be a performance problem.