Samething here. Works fine under Windows, and was working ok under Ubuntu 09.10. Very slow since I upgraded to Ubuntu 10.04 (Windows still ok).
Using a lenovo T500 with "03:00.0 Network controller: Intel Corporation Wireless WiFi Link 5300"
Using "802.11n only", "802.11g only" or "802.11a only" in either 2.4Ghz or 5Ghz doesn't change anything.
Disabling security doesn't help either.
I experience sometimes (very rarely) a sudden burst of speed, but it doesn't last.
Disabling + re-enabling WiFi with the hardware switch effectively brings speed up for some time.