I am not a Mac person, but I looked for mention of issues with slow WiFi speeds with a Time Capsulee on the Apple forums at http://discussions.apple.com
Two things were mentioned rather frequently there:
(1) assure that you have your wifi on the TC set to a broadcast channel as far away from strong interferring channels in your neighborhod. The respondees suggest using the software at http://istumbler.net/ to search for other wifi signals in your neighborhood that could be interferring.
(2) assure that you have both the 2.4 GHZ and 5 GHZ bands (they call it "dual N bands"?) turned on on the Time Capsule. Noting, however, that the iPhone will only be able to connect through the 2.4 GHZ band.
Hope this means something to you -- again, I am not a Mac person.