Firstly, this is my opinion and very much not that of my business etc etc. It's also based on my analysis and hunches, so you'll find quite a bit of 'original research' here.
Lion and ML are Apple's new way to do planned obsolescence. There's absolutely no way that an integrated GPU cannot process 3 multimeters, for example, as most don't know this, but an Intel HD 4000 is significantly faster than a GT120 (see: http://www.videocardbenchmark.net/video_lookup.php?gpu=GeForce+GT+120) due to the 120 meaning to be a HD 2000 beater.
I would say the Mac Pro 1,1 was fast enough to do 10 multimeters at once, in fact this is the first time I've ever heard someone try to say that a card is incapable of doing any kind of 2D graphics in this decade. It's not true, plain and simple. What is true is that Apple have reached a level where the majority of it's common consumers will not benefit from more power and therefore will feel no reason to upgrade (other than feeling cool about new toys). The only way around it is to make them feel the benefits, and the easiest way to do that is to bloat their code into making their software feel just a little sluggish. Sluggish enough to justify that shiny new MBP they've been eyeing up.
They've done it with the iPod an massive amount of times on the hardware front, allowing for parts designed to fail weeks after warranty. That's why yearly refreshes come out. You're stuff breaks and theres a new one that's conveniently just come out. Anyway. I digress, I think the bloating of Lion was meant to kill off a generation of Macs and they overdid it just a little, hence ML's slight speed boost.