Group Security Dashboard `summary` endpoint is timing out

When working, Group Security Dashboard history endpoint timing out , it was discovered that the summary endpoint is slow and times out. This is affecting adoption/usage of the Group Security dashboard.

In https://gitlab.com/gitlab-org/gitlab-ee/issues/11693, we added caching to speed up the history endpoint. It should be fairly straight-forward to take a similar approach with the summary endpoint.

Also note that we're heading towards First-class vulnerabilities, which may also solve this problem.

Over the past 4 months, we've seen the following timeouts (as of 6/13/2019):

vulnerabilities/summary - 20 occurrences