“ul li” always slower than “li” because it has to run multiple searches for the same result set
“form.class” generally faster than “.class” because it can run getelementsbytagname first and greatly narrow down its result set. Depending on your browser and library, this difference may almost be reduced to 0 by the implementation of getelementsbyclassname that has been implemented natively in some browsers.
“form#id” slower than “#id” because you’re running more searches than required by a simple getelementbyid

These are all scenarios that I’ve seen people bring up as test cases for other scenarios and apply the “well this is faster so that must be faster” logic, without understanding what’s really going on…