This side effect is normal, because in Forest the Brush tool works in a very different way compared with other scattering packages: the Paint areas are parametric. That means that once painted you can modify parameters as density, distribution, type of plants etc.
Instead to place the items directly, Forest "paints" a spline on-the-fly, which is used as scattering region. For large areas this may cause lag, because the spline must be rebuild with each stroke.
There are several workarounds:
- As you pointed, create several Paint areas.
- Once completed, you can convert them to splines, edit or attach all together ("Convert to Spline" button).
- Alternatively, use the Custom Edit mode (Tree Editor rollout). It lets to add and edit items individually (although not a Brush tool), in a similar way to other packages. This mode has not lag, but you loose the benefits of the parametric edition.