Some websites have dozens and dozens of tracking bugs and libraries, slowing downloads and delaying rendering. If you block them in Firefox, researchers demonstrated, some pages load much faster.
The paper shows that with Tracking Protection enabled, not only did Chew and Kontaxis see a 67 percent reduction in cookies set in the Alexa Top 200 news sites, but page load times were reduced by a median 44 percent, and overall data usage was reduced by 39 percent. Even if you aren't too concerned about privacy, that speed increase alone might be enough of a reason to enable the feature.
The speed boost comes from blocking requests to tracking domains, so it won't speed up browsing across the board, but considering the amount of sites that use some sort of tracking, the benefit should be fairly noticeable. Currently, Tracking Protection isn't turned on by default, as Mozilla is still gathering feedback about how the feature works, but it's fairly easy to enable the feature.