the thing with domain names is when you own one, you can decide where ANYTHING that uses that suffix goes to. Most importantly, these can be different IP addresses.
So
www.mydomain.com may point to your web server located in the basement of the small shop that does web page management for you and others. And mail.mydomain.com may point to a machine in other shop that's helping you with your email, or maybe to one of google's mail servers. And vpn.mydomain.com points to the vpn appliance at your business that lets you remote into your DB server from home. So all three of these use mydomain.com but all three have different IP addresses.
This makes it possible to put your mailserver on a different box, or even a completely different IP address / zip code. This has always been the case with domain names.
Mailservers have a little additional twist to them, that also ties into your domain name. "MX records" are attributes of your domain that tells any mailserver in the world where your mailserver is at. (since it doesn't HAVE to be
mail.mydomain.com) When someone sends mail to dude@mydomain.com, their mailserver looks up mydomain.com and reads its MX record, and connects to the ip address it specifies, to drop off your mail. When google hosts your mail, you tell your registrar to point your MX record to the server they tell you to use, and it takes over from there. Instead of mail being delivered to your main box, its delivered to one of google's servers, waiting for you to pick it up there instead.
The delay could be caused by any number of things. First thing to check is simple, just look in the email client under Accounts and see how frequently it's checking for new mail. Unfortunately the other causes are probably mostly beyond your control and have to do with gmail itself. If you're getting the delays with your
local mailserver, you really need a geek to stop by and poke around a bit.
