Yeah, OOK transmitters are slow as hell, that's expected.
It's not the frequency that determines that, but the modulation scheme and sophistication of the hardware. OOK is pretty primitive, and the hardware is garbage too (those are the crappy kind of receivers too - not that radiohead will recognize and take advantage of nicer OOK transmitter or receiver modules; I don't use that so I don't know if they have an option to increase the speed if you're using nicer receiver and transmitter).
the NRF24's are probably better, but I think they're still fairly slow. Neither of these are meant for high speed.