Since the start of this year, there is a new concept in the Discord API: Intents. These intents tell Discord which events your bot receives. Without intents, just presence updates might take up the vast majority of events you receive, and they aren't even useful to most bots! As a fix for that, you could disable guild subscriptions for a while now, but that solution has been superceded by the new intents system.
But with the new intent system, you can just tell Discord that you don't care about such events when your bot connects, and Discord won't send them to you.
Before intents, JDA would chunk guilds before any other events on that guild are executed by default. That just means that JDA sends a request via the gateway to retrieve all members in a guild when loading, and store them in its cache.
After that JDA would just keep the cache updated by looking at what events it receives. If there's an event indicating that a member up