Tub Drain SLOWLY Drains – If You Hold “Knob” Down It Drains At Regular Speed

I am sure this is just due to age and wear and tear on the drain, but I do not know where to start. My drain slowly drains, but if I hold the "knob" down it will drain at regular speed.

What should be my steps to replace parts in order for the drain to drain regular speed all the time?

EDIT
I pulled out the "knob" all the way out and the spring appears to be in tact as well as all connections. When out of the wall, I can lift the "knob" up and down with no issue and the "weight" (or whatever you'd call it at the bottom) moves freely. Surprisingly, there wasn't even that much hair or gunk built up on anything.

See this image, does someone notice anything off?