Try checking the voltage at the battery terminals while the slow cranking is happening, then have someone load test the battery to drop it to the same voltage. This test will tell you how much current the starter is drawing on a hot restart attempt. If it's much over 250-300 amps with a good-condition starter, you've got excessive drag inside the engine, maybe pistons expanded tight against the cylinder walls, etc.
Another good check for a too-tight rotating assembly would be to warm the engine up, and quickly remove the spark plugs to relieve the compression. If it cranks slow with the plugs removed, start looking for some kind of internal drag. The suggestion about running the starter with the ignition turned off, or even with the coil wire removed from the distributor cap will tell you whether or not you're dealing with a timing issue- - - -no spark, no kickback.
Jerry
