ReadyToRun images are not as efficient for .NET Core 3 as NGen for .NET Framework

* .NET Core Version: 3.0 Preview 8
* Windows version: 1809
* Does the bug reproduce also in WPF for .NET Framework 4.8?: No
* Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? No

**Problem description:**
Since Ngen.exe and Native Image Task are not available in .NET Core 3, ReadyToRun images are the only way to reduce the application startup time and avoid delays caused by JIT compilation. However, our tests show that for the same WPF application that initializes three times faster after using Ngen.exe, ReadyToRun images provide the difference of only 25%.
This is a huge hit for our users, since with many libraries and theme resources, JIT compilation is one of the main factors affecting load times. For some real-life applications, the use of Ngen.exe allowed our users to shave up to 6 seconds (half) off the initial start time.

**Minimal repro:**
You can find samples for .NET Core 3 and .NET Framework here: https://github.com/AlexChuev/ReadyToRunPerformanceTest
These samples use DevExpress WPF assemblies to demonstrate how Ngen.exe and ReadyToRun images affect projects with many classes and theme resources. If you need published apps or other samples for your tests, please let me know.

P.S. I'm posting this in the WPF repo because the difference between Ngen.exe and ReadyToRun images highly affects WPF applications. Normally, the JIT compiler processes classes and methods only when they are needed for the program execution. However, when WPF loads theme resources for a control, this causes all classes referenced in these resources (including classes referenced in currently unused or invisible parts) to be processed by the JIT compiler. In addition, static constructors of many WPF classes contain the DependencyProperty registration code that may cause even more classes and methods to be JITted.