When running large instances with a large number of issues (starts with several millions), calling /api/issues/search with the facet parameter can quickly takes a few seconds.
The same call without the facet parameter takes only a few hundreds milliseconds or less.
While this situation on the overall issues page could be understood as being caused by how big the large number of issues, this excuse does not exist when the situation is also present on the issues page of a project with only a few hundred issues.
This situation was also present on SonarCloud. Investigation were conducted and the problem was identified (poorly forged ES search requests) and fixed (see SC-1226).