DM Genie can become slow to load for large campaigns. Here are a few tips to help improve overall performance:
Clean up creatures that will no longer be required – like those from random encounters for example. The Group Manager 's "Edit Group->Delete ungrouped creatures" menu can do this automatically.
If you have completed an adventure module, consider exporting it (as a backup) and deleting it from your campaign.
Disable the 'Auto-generate Psionic Items' option (File->Options menu) if you are running a non-psionic campaign.
Remove unneeded campaign data – use the Custom Data Editor to easily filter by source.