I’ve been asked several times about this because I put a question on the practice exam related to it.
In retrospect, I wish that I hadn’t used the class IB as the example on this question and I will change it next year. However, for the record, all of the class I anti-arrhythmics block Na channels and blocking Na channels slows the action potential propagation. I admit that the effect is nowhere near as strong with the IB as it is in the IC and IA because the IB are open channel blockers that aren’t as effective during the rapid upstroke. Nevertheless, it will always happen to some extent and it is an effect which you may see particularly in diseased tissue.