This is literally slow cranking, not taking longer to crank. It sounds like you have an almost dead battery.
It occurs because the centrifigal advance is sticking (advanced), and you get pre-detonation against the starter. If it ain't doing this, don't worry about it.
If it is, you should be able to have someone crank the car and get a look at the timing if it turns over a couple of times.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe