After experimenting a bit further after reading Cuberto's findings, it seems that this may be a bug with Chrome and dual monitor setups.
As Cuberto suggested, I tried resizing my window a bit smaller, and this made the FPS shoot back up. Even if didn't have the screen maximised, but still had it spanning most of the screen it would be at max FPS.
Curiously, I disabled my second monitor and then put Chrome back into the maximised state, and it was fine.
As soon as I re-enabled my second monitor the FPS shot down again, so it would appear this is the problem.
Upd: it seems someone else has had this same problem with dual monitors (see comments): Why would Chrome cap frame rate at 30fps?