$ time ld.lld <omit> -O0 -o bin/clang-4.0
$ time cp bin/clang-4.0 foo
Whereas I have not done your specific test, I know that for the file sizes of executables I deal with in everyday work (around 10MB), the amount of time I wait for linking is woefully disproportionate.
A debug build of clang is normally hundreds of megabytes (~600mb IIRC, and normal linkers go bonkers dealing with it), so if LLD is actually only 2.5x slower than 'cp' at -O0, that's quite good I think.
The next question is how much memory LLD uses vs the competition in this test...