I do not know the sample time of the ADC0808, but assuming that it is parrallel and fast enough, you should be able to process in about 25 microseconds. )*)* seems to indicate an 8 bit a/d, why not use a '51 derivative with one built in.

What can make you too slow
1) using a RTOS.
2) writing in "real C" (C written with no concern for the processor architecture).
3) using floating point arithmetic.
4) plain old sloppy design.
5) plain old sloppy code.
6) using a serial access D/D
7) delays on the analog side of the ckt.
8) several other things that do not come to mind right now.

Erik