wc -l versus sed -n '$=' for a 20 million line file

$ /usr/bin/time wc -l wc_test.txt
20000000 wc_test.txt
0.36user 0.19system 0:06.28elapsed 8%CPU (0avgtext+0avgdata 2320maxresident)k
507888inputs+0outputs (1major+189minor)pagefaults 0swaps


$ /usr/bin/time sed -n '$=' wc_test.txt
20000000
3.08user 0.18system 0:06.26elapsed 52%CPU (0avgtext+0avgdata 2752maxresident)k
507824inputs+0outputs (0major+227minor)pagefaults 0swaps