(Probably a lot of work. Solving it reasonably likely requires drastic changes).

The current ranking caching strategy is not suitable for contest with many (e.g > 1000) users. It takes around 30s to generate such a ranking and the current RANKING_CACHE_TIMEOUT is set to 30s. Increasing this constant will not solve the problem, as the ranking gets invalidated every time some new solution is judged.

One of the possible solution could be introducing ranking_cache_timeout in the contest controller, for example 30 mins for contests with many users and 30s for ACM style contests. It is also worth considering to cache rankings separately for contest admins and regular contestants.

This issue is related to displaying links to "my submissions" in the ranking view. Apparently they get broken when the ranking caching with large timeout is enabled. Maybe this feature should be disabled.