How can I slow down the drawing of lines

There is no way to "slow down" the existing drawing functions.

So, you're going to have to come up with your own line drawing algorithm, like "Bresenhams line algorithm". You'll be calculating the position of each pixel in the line and drawing them yourself. But, this will NOT be so straight forward as you also have to understand how Windows interacts with your app to draw stuff. You'll have to keep track of where you are in the line drawing algorithm, like what you've drawn and current position and values, and draw SOME pixels in the line, then wait for your Paint method to be called again, draw some more pixels, and repeat until complete. Failure to understand this and properly implement it will just result in the entire line being drawn and shown on the screen all at once.