The default is to keep 25 messages per folder up to date (with read/unread state, etc.), checking mail every 15 minutes.
If you're going to increase "message to sync", just don't jump to 1000 or 5000 right away.
If any of those accounts are Gmail (Fastmail / Cyrus), then you'll benefit from installing the beta versions (either one), which can quickly decide if there are no changes in any of an account's folder and move on.
