Despite assurances from some clown back in 2013 that "LESS support could be implemented in a manner that does not degrade performance", lessphp makes up fully 40% of load.php's CPU time, which is unacceptable.
We clearly have to be a lot more aggressive about caching. The difficulty is that lessphp compiles LESS sheets in a single pass, so there is no invariant intermediate representation that could be cached and reused.
https://github.com/oyejorge/less.php seems like a healthier project overall, has a lessc compatibility mode, and does support caching of parse trees. We should evaluate it.