currently there is not build flag to statically link libc manually (it is planned though). Regarding startup time, it all comes to ASDF:
$ time ecl --eval "(quit)"
0.052 secs
$ time ecl --eval "(require 'asdf)" --eval "(quit)"
;;; Loading #P".../lib64/ecl-21.2.1/asdf.fas"
0.301 secs