My guess of an explanation is that as its turning a: the battery voltage drops so it slows, and b: compression builds up also causing a slowing. Then it eventually fires and off you go.
If it was siesed then I suggest you replace the plugs / dizzy cap / rotor etc, also remove the carb pistons and gently polish the needles (they often get a white powder build up on) use petrol or brasso (but be very gentle, don't want to remove any metal from the needle).
Once you get it running, I would warm it right up gently, then rev it up a bit, just to get oil pushed right round the engine.
Hope this helps
Richard
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