Here's a small sequence of Bob Pettit clips that demonstrate this.
Notice how to any untrained eye that has been told 50's and 60's players were 'slow' the unaltered footage would almost certainly be interpreted as 'slow athletes' rather than slow footage. Adjusting by using the shot clock however, reveals that the athletes move like.. well... perfectly capable athletes should. Now, imagine the effect this has had on people who for the past 40 years have only ever seen footage like this at an unaltered speed. People have literally grown up never seeing most of the players from that era playing the game at game speed.