Raoul might have a more in-depth response, but I have almost 1k songs in my library, and the SQLite database is less than 2MB in size, so I doubt that it's an I/O issue.
I would start by reindexing the songs (Tools menu --> Songs --> Re-Index Songs).
If that doesn't help, be sure the system is on an SSD, and has plenty of drive space and RAM available. If your OpenLP data folder is on a network drive or in the cloud, configure it so that it at least caches the data locally (e.g., for a Google Drive folder, enable offline access).
