Question

How much does less.js impact the speed of a site?

For standardization purposes, let’s assume we’re using Benchmark.less.

How much slower is the compilation process than the pre-compiled css? Also, at what filesize will users see a performance impact?

Answer 1

You can check load time with Firebug (you will exactly see what takes how long to load).

Additionally there is still the option that you compile your .less to .css locally or even do it online before you put it on the site.

EDIT:

Here is what I get trying benchmark.less on my local server:

less: parsed hxxp://localhost/testing/css/style.less successfully.
less: saving hxxp://localhost/testing/css/style.less to cache.
less: css for hxxp://localhost/testing/css/style.less generated in 640ms
less: css generated in 641ms

p.s. No matter what the load impact is, I would still compile it to css before you put it online, otherwhise people with JS disabled will get nothing.