Keeping a ast.NodeVisitor compatible API may have performance limitations

[cabiad]

See #1 (comment)

Another problem with the NodeVisitor API is that it couples registering a visitor subclass (batch of related node visitors) to walking (and visiting) the AST.

It's not clear to me whether re-walking each AST for each subclass would scale reasonably if we had dozens or hundreds of registered subclasses. In other words, would we linearly increase total execution time for a given set of ASTs or would some level of caching dominate, making the subsequent AST walks negligible.

If we see performance problems, a clear optimization path to consider is to walk once and call each visitor (method) registered for that node type

[cabiad]

Flame graph suggests that astroid.parse() is the primary slowdown. Walking the tree multiple times is not the slowest part of the process.