Compared to the 10 years he spent cultivating the first mortal coil, it seems like he was slow in the second mortal coil, but he actually wasn't.
Other people spend an average of 50 years or a maximum of 100 years at the second mortal coil, but he spent 20 years. That's 40% of the average time or 20% of the maximum time. So his progress can't be considered slow.
His progress certainly can't be considered slow when it is compared to the progress of his mother. His mother is currently at the sixth transformation, working to tackle the hurdle of the seventh transformation.
She has been working on the second mortal coil for 40 years now. He reached the peak of the second mortal coil in half the time, so he can't be considered slow by any means.
