dick greenhaus has hit on an important matter. Most guitar players mash down on the strings a whole lot harder than they need to, or should. Try keeping your hand light. Press only hard enough to get a clear tone.

I hate to suggest it because most folkies have a canniption at the mere idea, but it helps a great deal to practice scales while concentrating on keeping your finger pressure light. After you do it for awhile, you'll find that your left hand is a lot freer and faster.

This assumes a guitar with an action that has been set up properly, which is another issue that sometimes needs attention.

Don Firth