You're going faster, but the fatigue factor is rising even more rapidly due to decreasing efficiency. On a fixed-gear trainer, the only way to maintain a quick tempo is to become smoother. Pure form becomes the only 'fix' for an addiction to speed. Direct-drive compels you to actively manage continually moving crankarms. This responsibility can be frightening on a downslope.
Gravity provides propulsion.
You have the opportunity to passively experience an ultrahigh turnover rate. Let the good times roll. The adventure of relying on the universal force for effortless cadence is available only through a fixed-gear descent. Nothing else offers so much RPM with so litte exertion. You can focus on relaxing, and learn how it feels to get out of your own way.