[Timebox] Investigation sync-metadata performances

[Weiko]

Context

Sync-metadata command can take a lot of time and we need to identify the bottlenecks and improve it. DB and cache access, migration generation, complexity (we are looping over the same list multiple times, parsing could be improved) . Also health-check is quite time consuming and is meaningless right now, we can start by removing it if performances increase a lot and take the time to improve it later on when it become relevant again (right now, this code is so behind that we ignore the warnings).