Replacing thick tire with thin tire
The ability to use a thinner tire will depend on your wheel's interior rim width. The inner rim width will constrain how thin of a tire you can have. Take a look at the conversion chart below to see if you can use a thinner tire. (note this chart is conservative, many cyclist have gone thinner with no problems).



Thinner tires have less roll resistance, so that will help with the slowness. The flip side is it'll feel bumpier (but this is only noticeable at higher psi: 80+).

If you're interested to learn more, take a look at some material from the great Sheldon Brown: Tires &amp; Tire Sizing