After looking at the problem a bit more, we've seen that the delay happens when there's more of 2 "free" agents in a queue. And it seems exponential: more free agents, more time it will take to show the ringing status of the extensions (even if we hang up the call, the call will appear and then disappear).
Don't know if it's relevant, but looking at the fop2 log, when the call enters the queue, we can see the RINGING (output with a few greps to only show relevant data):
127.0.0.1 <- Device: Queue:127
127.0.0.1 <- State: RINGING
127.0.0.1 <- Privilege: call,all
127.0.0.1 <- Device: SIP/350
127.0.0.1 <- State: RINGING
127.0.0.1 <- Privilege: call,all
127.0.0.1 <- Device: SIP/351
127.0.0.1 <- State: RINGING
127.0.0.1 <- Privilege: call,all
127.0.0.1 <- Device: SIP/352
127.0.0.1 <- State: RINGING
....
But until we see the ChannelStateDesc: Ringing, the buttons do not appear as ringing. Don't really know if it could be a clue.