That's never going to be possible. No cassette player is going to be able to read a cassette at 15x normal speed without a loss of quality. Tape-based media can generally only be read properly in real time, so it will take you 90 mins to convert 90 mins. If you're just converting voice rather than complex sound (like music) then you may be able to get away with recording at 2x speed and then halving the speed in software after recording. However this isn't going to work at 15x, as you just won't capture enough audio data to get a good result.
If you can run more than one device in parallel you can convert more than one cassette in 90 minutes.