First time loading times IOS too long

Hi there,
I have successfully distributed my app via testflight ios to many beta testers. But there is one strange issue: Many people told me that the initial loading time is tooo long. So I‘ve tested the build on my iphone 8. What I‘ve noticed:
After first installation and starting it takes around 8s for it to start (black screen after splashscreen). But after that first start it takes just a second (like it should). Does anyone have an idea why and what to do about it?