Oh guess what: I didn't knock on wood hard enough. My fault for just using the expression and not actually doing it.
The case is: the browser is only lightning fast in a newly booted, empty Reason rack. In a normal project (or in an empty project that is opened after closing a filled rack!) it gets slow and sluggish as usual again.
Dear Propellerheads, I can only call this a bug/technical error in Reason
It seems like the browser IS capable of fast loading, except if it is held back by something. If it is the fact that the rack is filled that's holding it back, it shouldn't have problems in an empty rack that's opened after a full rack has been closed. Seems like the browser gets 'infected' by full racks. Hence this is a bug.