Daniel's got the meat of it covered.
As a driver gets older, the more difficult it is for them to improve. At 40 years old, his progress for each time you click 'Train' will be an incredibly slow crawl.
Training your driver, however, shouldn't be consuming 17% health. The number of stars your team's doctor has relative to your manager level affects how efficiently a driver is able to train.
Here's Jack's announcement from around 3 months ago about this:
For faster progress, keep an eye out for younger drivers that you can hire. Ideally someone with a low signing fee as they will need to be replaced rather quickly as you level up from the lower levels. The younger they are (lowest age is 17), the more progress they make each time they train.