btw, for reference with respect to the changes @tomoyafujita did: No single call is to blame. The main issue is that, for every single timer-invocation or message, the whole internal representation is traversed. In contrast, if you use the middleware directly, you can attach a listener directly to each communication objects. This avoids traversal completely.
However, the listener approach has the problem that we have very little control over when which message is being executed. That’s precisely why ROS 2 adds executors, and can even have different ones.
IMHO, it would help to look at the interface between rmw and the executor, to pass more information across and thus avoid traversal.