My tip to improve your typing speed is to have a good posture don't be all slouchy. If you are slouched then your fingers will be uncomfortable and nto be able to stretch your fingers when typing.

What really helped me improve my typing speed is by having a good typing stance.(sitting strait up comfortably)

When my fingers hurt after too much time on the keyboard, I just take a few minutes maybe get a drink of water and then stretch my fingers then get back to typing.