Description of problem:

When using e. g., 'dnf repoquery --requires --resolve <package> dnf is seemingly much slower than yum. A simple example on a i5 with 4 MB ram with warm caches:

$ time dnf repoquery -C --resolve --requires python fedora-reviev
[snip]

real	1m59.434s
user	1m58.479s
sys	0m0.968s

$time repoquery -C --resolve --requires python fedora-review
[snip]

real	0m3.010s
user	0m0.571s
sys	0m0.167s

This hits fedora-review, see e. g., bug #1275275. Given the figures, some kind of optimization should be possible (?)