[Serusers] ser start up

Hi, I have a situation where the SER is only doing registration and proxy, where else ASTERISK is handling all the call switching behind (UA to UA as well as UA to pstn) I am combining registration and proxy to be handled by one SER, only handling around 650 users at this stage. However when I reboot the server and start SER, it takes around 10 minutes just for binding user and then handle registration, during that period, no UA can make any outgoing call though they are already registered. After the registration is settling down, only then UA can make outgoing call. And after that, periodically, there will be a time where the SER is very busy with registration and no outgoing call can be made again. Most of the UA expiry value is 20 minutes. Is that normal? Or which area I shall look into? Is it advisable to separate registration and proxy into 2 SER? Thanks in advance for the advice. Regards, Chia