Have you gone through this thread already? Performance Q&A guide
Also, what plan are you on? Are you monitoring usage? Do you have additional server capacity units enabled?
As for database speed, especially with large lists the more constraints you can apply when you make ‘do search for’ requests, the faster the data will be served but that is all in the perfomance thread too.
it’s hard to give straightforward tips without seeing the app and structure. I can say, however, that your site seems to be down 