“optimizeLegibility is Decadent and Depraved”
http://bocoup.com/weblog/text-rendering/

What do you think?

The first thing I notice is the nice type ;-) 
I don't know how long it will take the wizards to nullify the noticeable speed hit of "Optimize" but I trust that they will.

Nicely written and concise!

I'm curious to see if the same issues hold true when using font-feature-settings to enable kerning and ligatures instead. 

I replicated the Moby Dick test that Nick mentioned in the comments on that site and also tested it with my own site. I couldn’t find a significant difference for a) OptimizeSpeed b) OptimizeLegibility and c) font-feature-setting kern/liga. 

However, it might really depend on the actual site. Certain layouts might lead to very different results.