I've registered 40044 host in a Satellite 6.3 snap 18 and a GET on /api/hosts took only 3 s to complete, including network latency from Brazil to USA. So I'm moving this BZ to VERIFIED.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA.
For information on the advisory, and where to find the updated files, follow the link below.
If the solution does not work for you, open a new bug report.
https://access.redhat.com/errata/RHSA-2018:0336