In regards to your latter comment, I agree. I think that that is very important and we should do what we can, to try and bring some answers to those kinds of questions.Well, I guess this is in part because speed is one of the things that are easy to measure. A tester can only use a limited amount of sites, so getting a truly representative sample of the "blocking power" is difficult. What remains are speed and customization options.
One of the most important aspects IMO hasn't been addressed at all so far: How well does a blocker stop invisible trackers? Many devs will focus on the obvious results, so it's possible that some blockers let more trackers through than they should. It's difficult but not impossible to test (e.g. one could set up an HTTP proxy or use Wireshark to see if there are still requests going out for well known trackers).
As to speed, I am not saying it isn't important. It does factor into the app equation. I just don't think it should be the primary focus that many people appear to look to.