Furnace fan starts slow and is getting slower
Sounds like the capacitor is dead/dying.

If you have a capacitor tester, or a multimeter with a capacitor test feature, you can test the capacitor. There should be a micro farad rating on the capacitor (e,g, 10µF), and a variance percentage. When you test the capacitor, it should be withing the variance percentage. If it's not, then it's bad.  For example a capacitor rated 10µF ±6%, should test between 9.4 and 10.6 micro farad. If not, then it should be replaced.