are you loading all the files in one spot? Are there many files?
are you running in release mode (sorry never used Xamarin but i suppose that’s an option there aswell)?
is there any cpu intensive operation happening on load/init?
is this happening 'cos you are building the project everytime?
you need to offer more info on your problem, otherwise it is impossible to find the answer.
Try switching all your textures to PNG files loaded with Texture2D.FromStream. This is the fastest way to load them, unfortunately it may hurt performance, or have problems on devices with less graphics memory.
The startup is extremely slow, even if I create a blank project without any files in it.
I don’t think it has something to do with the textures or the content at all.
It affects only MonoGame, my other C# applications don’t have that problem.
Use a profiler to see where the CPU cycles are spend.
If the problem is mono startup use the dx project on windows.
If it is your initialization code it is up to you to optimize the function that takes up more time.
If it is the content manager try to DXT compress your textures, use lower resolution when possible, use lower bit rate for audio/music.