Assuming the same sort of incremental advance in AI that we've seen for decades, this is borderline tautological. The first AGIs will likely be significantly dumber than humans. I would be hard-pressed to imagine a world where we make a superhuman AGI before we make a chimp-level AGI.
Note that this doesn't disprove an intelligence explosion, merely implies that it won't happen over a weekend. IMO, it'll certainly take years, probably decades. (I know that's not the prevailing thought around here, but I think that's because the LW crowd is a bit too enamoured with the idea of working on The Most Important Problem In The World, and gives insufficient respect to the fact that a computer is not merely a piece of software that can self-modify billions of times a second, but is also hardware, and will likely have that incredible processing speed already fully tapped in order to create the human-level intelligence in the first place)
I think you underestimate the degree to which a comparatively slow FOOM (years) is considered plausible around here.
wrt the Most Important Problem In The World, the arguments for UFAI are not dependent on a fast intelligence explosion - in fact, many of the key players actually working on the problem are very uncertain about the speed of FOOM, more so than, say, they were when the Sequences were written.
Claim: The first human-level AIs are not likely to undergo an intelligence explosion.
1) Brains have a ton of computational power: ~86 billion neurons and trillions of connections between them. Unless there's a "shortcut" to intelligence, we won't be able to efficiently simulate a brain for a long time. http://io9.com/this-computer-took-40-minutes-to-simulate-one-second-of-1043288954 describes one of the largest computers in the world simulating 1s of brain activity in 40m (i.e. this "AI" would think 2400 times slower than you or me). The first AIs are not likely to be fast thinkers.
2) Being able to read your own source code does not mean you can self-modify. You know that you're made of DNA. You can even get your own "source code" for a few thousand dollars. No humans have successfully self-modified into an intelligence explosion; the idea seems laughable.
3) Self-improvement is not like compound interest: if an AI comes up with an idea to modify it's source code to make it smarter, that doesn't automatically mean it will have a new idea tomorrow. In fact, as it picks off low-hanging fruit, new ideas will probably be harder and harder to think of. There's no guarantee that "how smart the AI is" will keep up with "how hard it is to think of ways to make the AI smarter"; to me, it seems very unlikely.