Yep!.
According to the specs, 4.4fps in full frame mode, 6.4fps in APS-C crop mode.
The reason will be the buffer fill/write rate.
Full frame mode has more data per images(eg. 70-90MB / image), APS-C mode more likely to be 25-30ish MB / image .. so for every 6.4 APS-C images per second, you have less total Mb to write to the card every second.
One thing I like about Pentax .. is their philosophy that the customer is king!
You can update you K1(series 1 model) to a K2-II model for a reasonable sounding fee(I think about $500 or so).
It takes a while to do, as they change physical bits internally AND subsequently label your series 1 model as a -II model too.
A very cheap update