In the extreme case of block that takes 1 hour to validate, there could be six competing blocks at the same height (i.e. same total work). But generally for every second it takes to validate a block, there’s a ~1/600 chance of a competing block appearing. I just don’t know it that’s enough to deter such an attack though.
It’s certainly a stronger measure to slow-block attacks. But it would also significantly change behaviour in a non-attack scenario. E.g. single confirmation would be a lot less safe.
And when a miner sees a new valid block, they have to choose between ignoring it or mining on top of it, based on some estimate of how likely it is another miner would produce a faster-to-validate block. This seems like a can of worms. (though my proposal may have a similar worm)