Tip 02: Stamina
When your Stamina level is down, you will be slowing down as you are too tired, in that case stop running (release the Shift key).
The warning red icon on the bottom left of your screen will show you when to stop pressing the Shift key.
It is the same underwater, if you swim fast for too long you will lose health as your stamina will be down.
Check the oxigen bar on the top of your screen, when it reatches the stamina icon, then you can swim faster by pressing Shift.