That long running step is your initial data synchronization.
Are you syncing more data than you really need? Also if you have large images or documents, these will all be synced the first time. Mendix checks whether file documents and images are changed so will not download them again to the device unless a new file was uploaded.
Also, usually the user does not need all data on the device, usually only a part of the object lifecycle. Setting up XPaths on the sync config of your native navigation profile can help reduce the amount of data synchronized with the device.
If you need to sync a lot of master data, I created a pattern for that, watch the video about it: