Per DSpace 7 meeting on July 18, early performance testing of the Top Communities REST endpoint shows that is slower than expected. This may simply be because more information is being returned than is needed (so it might be solvable via Projection as described in DS-3533). However, there also may be a way to take advantage of caching or similar to improve performance.
The early performance testing results of this endpoint can be found here: https://cwilper.github.io/dspace-perftest/report/2019-06-04/7.0-preview-1/details-big#big-a-home-requests
Specifically, this call takes over 2 seconds alone: