First of all, I'm a little confused as it said 12% when the page first launched and no Flagfox update was posted since then. I have no clue where that new +4% came from, and since it doesn't tell you
what is slowing down the startup, I can't use any of this info to make anything better.
Secondly, the page has a few problems; more precisely, its explanation is wrong. It says it's listing the addons with the largest startup slowdown,
which it does not. It left out a rather crucial bit of information that is only stated on the
blog post announcing the new list. The list is only out of the top 100 most used extensions, showing the 50 with the most startup time impact (10 until you click "more"). There are plenty of other addons that may have far worse of a performance impact on startup, but if they're not in the top 100 they're not listed at all there. This leads to the false implication that these on the list are all bad, whereas the average is around 10% (stated in the same blog post) and their stated ideal is around 5%. I filed a bug for the bad explanation on the page, but as of yet haven't had a reply.
As to what the startup impact actually is for Flagfox, even on an older computer it's not really noticeable. Mozilla is trying to point out the cumulative effect of installing lots of addons, each with a negligible impact but adding up to a larger issue. One of the other problems here is the difference between cold and warm startups. I'm pretty sure the list is measuring cold startups (startup right after boot of computer) and not warm startups (startup after previously having loaded before and still waiting in memory). Flagfox has a much faster warm startup than cold, as does everything, but most of the impact being measured here is in cold startup I suspect. (actually, for all I know it's measuring startup on first install, which would be quite wrong as that would count slowdown from the installation itself) Though, as I said above, they aren't actually giving me any real information on this, which is a bit annoying.
I'm fairly certain that even with some possible problems with the numbers that Flagfox may have an above average startup impact, but not a very large one. I did improve things a little bit in some past updates and I'd love to try and find ways to reduce it more, but I'm already following their stated "best practices" guidelines and have done my own testing to see that what I do on Flagfox startup doesn't impact much of anything. The big hit is probably coming from loading of the files themselves, not by anything in my code. I'd like to try a few things, but I can't for the life of me get consistent enough testing results to try new things and see what helps. In my testing, Firefox startup times vary so much that I can't measure anything, and testing cold startup in any remotely reliable way, i.e. rebooting constantly, is a nightmare. They say at some point they'll add the ability for developers to upload a version to run through the test they're using for their performance page, which would help, but that's not available yet.