I'm going to be a little nitpicky here: This would be grammar checking, not spell checking. "Slow" is spelled correctly, it's just not the right word (adjective vs adverb).

This also seems to still be in the experimentation phase and is not released as far as I can tell and may never actually make it into Chromium based on:

> In the future, Chrome may identify sites that typically load fast or slow for users with clear badging. This may take a number of forms and we plan to experiment with different options, to determine which provides the most value to our users.

Regardless, I agree. This probably should've been caught early.

Maybe they used adjectives instead of adverbs because it’s both ad-words and they can no longer feel the difference.

Your comment about "ad-words" made me think: could Google sell ad space on these "slow site" pages? Site X might jump at the chance to pay Google to advertise its service on site Y's slow site page. I'm only half-joking.