We are running a PPPoE server on a Mikrotik CCR1009-8G-1S (6.47.1) with around 220 PPPoE clients (half of the tunnels being NAT'd), most of them Ruckus access points. We've never had any problems until a month ago when we started noticing some PPPoE clients were taking too long to connect. Five, ten minutes. Sometimes even longer.
(we didn't have this problem when the number of PPPoE clients was below 200).
The only solution we found was to log into the Ruckus WAP and click on "renew" (under PPPoE). The PPPoE tunnel would then show in the active connections table in no time. Once connected, they don't experience any problems with speed.
CPU usage on the Mikrotik router is quite low, peaking at 30%. Bandwidth peaking at 400 Mbps but we have this problem even when there's no much traffic.