I've had a ficus ginseng bonsai for more than two years now. I've had some problems with the watering, which resulted in leaves yellowing and falling, and so I'm trying to be much more careful. However, some leaves, mostly big ones, are still yellowing but at the same time new leaves are sprouting very fast (which had never happened before). Am I doing something wrong?

New growth is good. You must be doing something right.

Yellowing CAN be a lack of fertilizer. But maybe you can post a picture and tell us how you care for it, where it is positioned etc?