A “Safety Alert for Operators” (SAFO 09004) from 2/11/09 says.
Slow the aircraft to a fast walking speed on the centerline of the landing runway prior to attempting to exit the runway. Taxi at a fast walking speed until parked at the ramp or until aligned with the centerline of the runway for takeoff.
Since it’s not clearly defined by the FAA, it’s assumed that most airlines limit taxi speeds via their Standard Operating Procedures (SOPs). Also, due to the variety of equipment, it might be unfeasible for the FAA to mandate a specific speed.
1. FAA: Safety Alert for Operations (SAFO) 09004, 2B. Retrieved from the original.