Hello, any query you make from Kamailio, you'll have to handle it yourself, so the load will be affected, of course. There are alternatives to use such as CGRates, or simply through reading JSON socket events. Also, avoid performing any procedures within the B2BUA, whether it's FreeSwitch or Asterisk. The more external your billing control is, the better performance you can achieve. Here's some material for you to check out:
https://github.com/cgrates/cgrates https://www.kamailio.org/events/2015-KamailioWorld/Day2/14-Dan.Bogos-Advance... https://www.youtube.com/watch?v=Hsvcwleb-fY&t=2s
This is my humble opinion, within what I can contribute ;).
regards