Probably a combination of several factors. Check battery ground to frame and engine and from starter to engine. Check all physical connections from battery to starter. Could also be dirty communtator or worn brushes or a combination of both. Don't forget to check battery specific gravity.
Everything Don said and check your battery cables, The original ground strap may be corroded away and also check the gauge cable. Many have been replaced with 12 cables are much to small of a gauge for 6 volt systems. Gauge 00 (2/00) will work fine for the positive cable. The weakest link in the starting system is the floor starter peddle, be sure the connections are clean.
Where Are You From? Fremont, CA
Do You own a car built by Durant? 1929 Durant Roadster & 1931 Durant Model 619 Sedan
On one of my old cars I removed the rust on the frame where the ground connection is located along with cleaning the cable. Applied lithium grease during the assembly and what a big difference it made to the crank rate!