If it feels "slow" when it gets dirty, you're probably putting on too much lube and it's collecting dirt. Clean it, lube with a dot of oil on each link, and wipe off the excess. Wipe the chain down after every ride but don't apply more lube.

If you do a little research, you'll find that improving drivetrain efficiency has little to do with overall speed. Sure, the bike will feel, sound, and shift better, but a 10% improvement in drivetrain efficiency will only make you a tiny bit faster. Have a look at the graphs in this article: https://www.sheldonbrown.com/rinard/aero/formulas.htm