The first time an app is opened it is loaded into memory. So if it is a big app with a lot of data it could take some time to load. If you are doing a lot of calculations in the app that could slow it down. If you are not loading a QVD file and are loading data directly that could slow it down. It could be that you don't have enough memory on your server.
As you can see it could be many things. If you share more details about your app and your situation that would help.