Condition has nothing to do with training speed. Age and value are the 2 things that you have to look at.

There are threads devoted to training, you should read a few.

18-22? is the primo age for fast training.
the higher the value, the odds are that the player will be a fast trainer.

ex. two 18 yr olds:
4* ST = $3.5M <---Faster trainer.
4* ST = $2.1M