Try creating a new document with no preset profile, one with RGB and another with cmyk. If the RGB document hangs, you may wabt to look/change at what color profile you are using for RGB.
Thanks for the tip! I tested out creating a new document with no preset profile, and it worked— no Selection Tool lag. I guess the problem I was encountering was only specifically with the "Video and Film" profile, so I'll just avoid using that. From now on, I'll stick to creating a custom profile when making a new document!