joema mentions that each library, event, project, dup project, and snapshot have their own SQLite database. He also mentions something like 10k clips per database, but that it depends on HW, drive space, and other factors...
I've observed that when you have more that roughly 30 snapshots in an event, opening that event may become slow. There is no hard limit, and that is just an observation I previously noticed -- don't use it as some kind of rule. It will vary depending on the hardware and whether the library and cache are on SSD.
Each project in each event (inc'l snapshots) is a separate SQLite database, consisting of a CurrentVersion.fcpevent file. Within that are eight SQL tables, with several indexes on some tables. Plus each event also has a separate event-wide database. E.g, clips not in a project may contain markers, ratings, keywords, edits, Fx, etc, and this must be stored somewhere.
Each database connection entails certain overhead, yet there is also latency in opening a database. Certain FCP search operations likely require numerous databases be opened, but the overhead in maintaining very many in an opened state may also have a performance cost. It appears that FCP will try to balance these two conflicting areas by deferring opening projects/snapshots in libraries (or events) with tons of events and projects. When you click on a project it may then try to open those in the background, causing a slowdown. FCP writes a brief status message saying something like "opening projects..." (I can't remember the exact wording, but it lingers for a few seconds). When that happens it may imply FCP has reached a state when opening the previously-deferred databases is now required.
The underlying database model is apparently an object-oriented "graph database" using Core Data, which in turn uses SQLite as the persistent data store. It appears FCP does not directly emit SQL statements to the lower database layer but Core Data translates graph database directives to produce SQL statements. Maybe there are limitations there.