Note that there are three separate issues:
1: how long does call setup take (from the time your ATA sends the call commands to the provider, until the far-end switch starts ringing the called party
2: how the providers are playing the ring-back tone
3: how long does it take from the time you stop keying in ("dialing") the phone number, until your ATA sends the commands to the ITSP
#1 varies by ITSP. I use Callcentric, and generally it is very fast. Some providers take longer.
#2: the ring-back tone is essentially fake. It's generated by the phone switches to give you that comfort level that stuff is happening to complete your call, and it has no direct 1:1 correspondence to the called party's phone ringing. So, the telcos can manipulate it, or even replace it with music, advertising, or whatever. In the classic POTS PSTN world, the ringback tone generally starts at the "correct" time, that is, when your called party's telco's phone switch is starting to ring their phone. In the old days (if you are over 55) you may recall that calls would take a noticeable amount of time to start ringing, while the switches were click-clacking and selecting trunks, etc. After the ancient analog switches were replaced by the digital SS7 switches, call setup was almost instant. In the SIP world, it can be manipulated.
#3 is tricky, as it depends on your "dial plan". SIP ATAs have two delays built-in, to wait and see if you are done entering digits, and those delays can be selectively eliminated via the dial plan string, and it depends on whether you're dialing 7, 10 or 11 digits, or some *xx command. Dial plans are tricky. There are some experts here on the forum who can help with editing yours, if that is the issue. Try entering a # key after you're done dialing to see if it speeds things up. "#" is like the Enter key; it means "I'm done entering characters, go ahead and process the string". If entering # helps, you've got a sub-optimal dial plan.