Slow step through and watch variable expansion when debugging in a Docker contianer

There's a performance difference between native debugging and Docker container debugging. The problem can be observed when expanding variables in VS Code. In this test project https://github.com/jamiegs/dotnet-debugging-performance the difference between native and docker is small but still visible. On a real code base, it is much more significant. See this video https://youtu.be/V_jWtOHjvOg for a demonstration.

The details here are on macOS + VS Code. But we have observed the same on Window + VS Code and Windows + VS. Run on high spec Mac Book Pros and Dell XPS hardware. Changes to resources allocated to the container does not seem to make a difference.

## Environment data
```
.NET Core SDK (reflecting any global.json):
Version: 2.1.300
Commit: adab45bf0c

Runtime Environment:
OS Name: Mac OS X
OS Version: 10.13
OS Platform: Darwin
RID: osx.10.13-x64
Base Path: /usr/local/share/dotnet/sdk/2.1.300/

Host (useful for support):
Version: 2.1.2
Commit: 811c3ce6c0

.NET Core SDKs installed:
1.0.1 [/usr/local/share/dotnet/sdk]
1.0.4 [/usr/local/share/dotnet/sdk]
1.1.4 [/usr/local/share/dotnet/sdk]
2.0.0 [/usr/local/share/dotnet/sdk]
2.0.3 [/usr/local/share/dotnet/sdk]
2.1.4 [/usr/local/share/dotnet/sdk]
2.1.300-preview2-008533 [/usr/local/share/dotnet/sdk]
2.1.300 [/usr/local/share/dotnet/sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.0-preview2-final [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.0 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.0-preview2-final [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.0 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 1.0.4 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 1.0.5 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.1 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.2 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.4 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.9 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.0 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.3 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.5 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.0-preview2-26406-04 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.0 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.2 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download
```
VS Code version:

1.25.1

C# Extension version:

1.15.2

## Steps to reproduce
Based on code in https://github.com/jamiegs/dotnet-debugging-performance

There are two Launch Methods, Docker and Native.

- Add breakpoint to 'return View()' line in Home Controller. HomeController.csd
- Run application
- When it hits breakpoint, expand the 'this' variable. Natively it takes about 3 seconds, Docker takes 5.
- - While this difference isn't that big, the difference grows dramaticly with larger applications.
- Also hitting 'Step over' or 'Step into' is slower in Docker vs Native.

## Expected behavior
- I would expect docker and native to have very little difference in debugging performance.

## Actual behavior
- The difference is noticable and increases with application size.

Demo video comparison using https://github.com/jamiegs/dotnet-debugging-performance
https://youtu.be/lkDnFPGOumM

Demo video comparison using a larger, 20,000 line application.
https://youtu.be/V_jWtOHjvOg