Rules and descriptions from earlier editions of the game have no standing in 5th edition D&D. Though related, they are separate and stand-alone games; though referring to previous editions may be helpful for historical context, the rules in them should not be considered to be valid in 5e.
So how fast does Tenser's Floating Disk go?
In the specific case you describe, the way the disc moves is quite clearly stated, in that it moves so that it is within 20 feet of you:
The disk is immobile while you are within 20 feet of it. If you move more than 20 feet away from it, the disk follows you so that it remains within 20 feet of you. It can move across uneven terrain, up or down stairs, slopes and the like, but it can't cross an elevation change of 10 feet or more. For example, the disk can't move across a 10-foot-deep pit, nor could it leave such a pit if it was created at the bottom.
If you move more than 100 feet from the disk (typically because it can't move around an obstacle to follow you), the spell ends.
If you move away from it, it moves towards you at the same speed; it will be going however fast it needs to go to ensure that you are not more than 20 feet away, so long as this does not violate its restrictions on sudden elevation changes. This applies whether you are moseying along at a sedate 30' speed, or you're rocketing away at hundreds of feet per round (as long as you're not teleporting more than 100' at a time). This is one of the examples of how things in 5e may behave very differently to earlier editions.