Tub Drain SLOWLY Drains - If You Hold "Knob" Down It Drains At Regular Speed
I assume you mean the "knob" is the drain open/closed lever located on the wall of the tub,.just above the drain, right?

If so, your most likely problem is that the drain mechanism is out of alignment. There is an "overflow drain" that runs down from behind the drain lever that you are holding, connects to the main tub drain, and continues out to whereever.

Inside that downward drain is a metal arm that connects to the tub drain open/close lever (the "knob"). When you move the drain lever up or down, it causes that metal arm to move up or down in opposition (It's like a seesaw, when this end goes up, the back end goes down.)

The metal arm is attached to a metal plug that will block the main tub drain, or release it.

You can unscrew the face plate behind the drain lever and pull out the face plate with the lever, the metal arm, and the plug. Once you have them out, you can almost certainly adjust the arm, changing the position of the plug up or down. That lets you set the position exactly right. 

It may be that your drain lever doesn't stay put any more. If so, you will need a new set- these things come in packages from your local home center. According to my smartphone app with the orange icon, they sell for $20 USD upwards, depending on color and style.