When you say "using a decoder from US Digital", do you mean one of the chips made by LSI Computer Systems?
Unless you happen to need extremely high precision in addition to speed, I would second the advice above to solve this mechanically rather than electronically. US Digital sells incremental encoders that support counts as low as 32 cycles per revolution, with shaft speeds as high as 10,000 RPM. If your rotating platform is >10,000 RPM, and you don't specifically need to know the direction, maybe just a singular magnet and a single hall-effect sensor could read the platform speed?