The video isn't online yet, but at FOSDEM this weekend there was a talk about "Firefox on Android". The guy said that it's actually adblock that's causing the long delays.
As a general rule: if you think Firefox is slow/laggy, try running it with addons disabled. One or more addons are often the culprit.
For page load lag, addons like ABP are often especially bad because they're running the URI of each request against a bunch of regex filters to determine if the load should be allowed to begin.
