I recently purchased a Time Capsule to replace an existing 802.11g router. The Time Capsule should have been much faster, but I had one machine on my network that was going extremely slow - to the point that it felt like dialup.
I talked with the Apple rep, and he suggested the usual things: change the channel, check for interference, etc. Nothing helped.
I did a lot of digging and what I finally found was that the Mac that was going very slowly had a setting in the Network system preferences for a DNS. It seems that the machine got that DNS setting from my old network, and when I ran the setup app for Time Capsule I used the option to "Replace Existing Network". It did that, but it didn't use the same IP addresses as my old one. So, the slow Mac was looking to that non-existent IP address as a DNS and it would never connect. It would finally time out and then continue on normally.
Moral here: If you are having problems with a new Time Capsule, it might not be the TC that is the issue. Check your Network System Preferences to see if there are other things in there that will slow you down.
24 inch iMac, Mac OS X (10.5.4), Got our first Mac in October, 1984