It depends on how many controls you are loading on startup and if the core libraries are already loaded.
If your app creates a lot of controls on the first screen then you have that time added to the load time. Look at the network tab using F12 on chrome to see what takes 10 seconds. Maybe the cache is turned off. Also look at how many controls you are creating at startup, if it’s about 100 then you start seeing some slow down because of browser rendering.
Let me know. And in case you have a link for us to try, send it privately to firstname.lastname@example.org.