ImageEx performance is very poor while loading lots of images at the same time

Describe the bug

If there is a page with lots of different images and load them use the ImageEx control, it will block the UI long time.

Steps to Reproduce

Steps to reproduce the behavior:

1. Create a blank page.
2. Add lots of ImageEx controls to the page and set the IsCacheEnabled to true.
3. Set different sources to each ImageEx control.
4. Navigate to this page and it will take very long time to navigate.

Expected behavior

Don't block the UI as much as possible.

Screenshots

Use the framework Image control: Use the toolkit ImageEx control:

Environment

NuGet Package(s): Microsoft.Toolkit.Uwp.UI.Controls

Package Version(s): 7.0.0-preview1

Windows 10 Build Number:

• Fall Creators Update (16299)
• April 2018 Update (17134)
• October 2018 Update (17763)
• May 2019 Update (18362)
• Insider Build (build number: )

App min and target version:

• Fall Creators Update (16299)
• April 2018 Update (17134)
• October 2018 Update (17763)
• May 2019 Update (18362)
• Insider Build (xxxxx)

Device form factor:

• Desktop
• Xbox
• Surface Hub
• IoT

Visual Studio

• 2017 (version: )
• 2019 (version: )
• 2019 Preview (version: )

Additional context

Test app source code is here: https://github.com/h82258652/ToolkitImageExPerformanceTest