Little torque of single phase asynchronous motor
This is the usual behaviour of asynchronous motors. See their torque-speed characteristic:



The acceleration depends on the difference between driving machine torque and working machine torque at any particular speed. If your working machine has a pretty high torque at low speeds, you may even run into this situation:



Here, you cannot even reach the green operating point. The machine is stuck on the left. You need a clutch to run this working machine with this driving machine.

Using a bigger starter cap may improve things a little bit but don't expect wonders. The problem isn't the cap but the machines you have.