This situation is annoying because it means you are being dragged into, and probably have to action, a conversation that has been happening without you for days

This situation is annoying because it means you are being dragged into, and probably have to action, a conversation that has been happening without you for days or weeks (or even months). The solution is to involve the right people in the conversation at the right time. The solution is most certainly not to just send an IM, which will lack all of the context that the email thread contains. People who think they are too important to read the work emails their colleagues send them are generally awful to work with.