I decided to add expiry policies to each queue so that the system cleans itself from stale messages and that fixed all the message dropping issues.

4.0 Changelogs state that they are switching to a new k/v storage (switching from experimental to default)

Mnesia stores vhosts, users, permissions, queue definitions and more. This is being transitioned to Khepri, which improves a lot of things (maybe most importantly netsplits) but not directly message speeds.