I'd like to see this backed up by numbers. Sure, Lion and Mountain Lion are quite slow on spinning platters, but OS X feels almost an order of magnitude faster since I had an SSD. Much faster than Snow Leopard with a hard disk. The numbers are also on my side, an SSD completely blows away hard disks both in access times and read/write speed.

but the implementation sucks on multiple monitors

Indeed. Big time. It works ok on the road, but when I have an external screen connected I never use full screen support. It would be easy to make things better: allow users to put another full-screen application (or desktop) on the secondary screen.

Tons of annoying little bugs at the lowest levels, and you really wonder what was broken there that they needed to fix. Like the mouse cursor that sometimes disappears.

I really disliked Lion for all its bugs. But things have been steadily improving since Mountain Lion, to the point where even iMessage works most of the time in 10.8.2 ;).