If a very long Conversation becomes slow to load, or if the topic naturally diverges to a new concern, consider branching off into a new Conversation. Both the old and new Conversations will have links to each other, so you can move between them easily. See How do I create a new Conversation from an old one? for more information.
If you have a long list of Conversations, here are some tips for managing them:
Use collections to gather related Conversations together, like those that focus on a common topic. Rather than scrolling through your Conversations list, you can go directly to your Collections list instead. See Collections
Mark the Conversations you really need to pay attention to as favorites (click the star next to the Conversation name), then use the Favorites filter on your Conversations list to focus on them (or click the star icon in the banner).
Use the Sort By menu at the top right of the Conversations list to arrange the Conversations alphabetically, by the date of the last update, and so on.