are saying the engine is "Turning Over" slowly, or it takes a while for it to start?
I would have to say both. once it starts I have to wait until it warms up and the RPMS ramp up before I start mowing the lawn with it.
Engine cranking slowly: Clean up all of the connections between the battery and the starter. And the starter to bell housing. Might be that the starter needs to be rebuilt. Also do a voltage drop test on the battery.
Slow starting: Complete tune up as the other have suggested. This is usually an ignition problem.
There are a couple of other things. Check for a vacuum leak at the carburator to manifold and manifold to engine block. This would cause a lack of power until warmed up.
Pretty far down the list is to check for a sticking or worn mechanical advance in the distributor.