I doubt it's a specific website, you'd be surprised how many we can add or remove in the space of a single database update, especially with the new malicious IPs we've been tracking for hosting exploits lately (typically many hundreds per day, and that has ramped up in the past couple of months so the timing fits).
Have you also run DPC Latency Checker on your computer with the latest MBAM database updates? When I run it on my work computer I also get the big red spikes with Website Blocking enabled and don't get them when I disable that feature. So I am 2/2 in terms of having the Website Blocking feature slow down my computers and would imagine that this problem is very universal. It's just that most people do not realize there is an issue without having a visual confirmation like a network TV recorder. I mean I wouldn't have known if I didn't get the pixelated video beginning in March.