There are many different versions of taiji form origination tales.
I found one recently that was compelling.
Tung Ying Chien and Yang Chen Fu collaborated on a "fast set" which was unfinished at
the time of Yang's death. Later Tung finished it. It is now taught by his grandsons Tung
Kai Ying and Dong Zeng Chen.
It has been claimed by some that Cheng-Fu invented the slow form. However here is a claim worthy of consideration that indicates that the fast form of Cheng-fu was his creation and the slow form was the transmission he recieved.
It is well known that Cheng-fu altered postures but how he altered pace is controversial subject.
Some claim that he took a fast form and slowed it down. This I do not believe, while it is true that many transmissions from Yang family members include fast forms in some cases: slow forms are also well conserved.
The credible source above indicates that among the alterations Cheng-fu was making to the from included an increase in pace, rather than a decrease in pace. Some claim that the fast pace is authentic and that the slow is not, however there are at least 3 accounts of where the Slow yang form came from.
They are as follows:
1 The slow form was invented by Cheng-fu who took a fast form and slowed it down.
2 The slow form was a secret Chen village form that was transmitted to Yang Luchan
2a the slow form is a modified version of a "slow" Chen village form that was split by Luchan into two forms, the Public and the Michuan, the slow long yang form being the Public version.
3 The slow form was transmitted to Luchan by Taoists after he sought the art that influenced Chen village, this version says that he was dissatisfied with Chen style and kept searching for the arts that it drew from and that he found them in the form of the same sect that the Chens had learned from. In other words this version says that Chen style is a combination of Taoist martial art and Shaolin, and that Luchan learned the Chen material and sought out the Taoist martial art which was transmitted to him by a disciple of Chang Sang Feng or even by CSF himself.
I think the least credible version is the first.