Verified on Satellite 6.3.0 snap 13 with 100k packages in several repos.
API call returns in ~500ms. Packages page in UI loads in 3-4 seconds.
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