RealProxy use is *dog slow* and impacts test run

[AArnott]

In analyzing why my tests were running so slowly, I found that 25% of the CPU time in the test runner process was lost to using RealProxy:

Name	Inc %	Inc
mscorlib.ni!RealProxy.PrivateInvoke	25.1	19,157
+ mscorlib.ni!RemotingProxy.Invoke	25.0	19,032
|+ mscorlib.ni!RemotingProxy.InternalInvoke	24.8	18,942
||+ mscorlib.ni!RemotingProxy.CallProcessMessage	24.8	18,930
|||+ mscorlib.ni!SmuggledMethodCallMessage.SmuggleIfPossible	15.7	11,958
||||+ mscorlib.ni!System.Runtime.Remoting.Messaging.SmuggledMethodCallMessage..ctor(System.Runtime.Remoting.Messaging.IMethodCallMessage)	15.6	11,904
|||||+ mscorlib.ni!MessageSmuggler.FixupArgs	11.5	8,770
||||||+ mscorlib.ni!MessageSmuggler.FixupArg	11.4	8,730
|||||||+ mscorlib.ni!RemotingServices.MarshalInternal	11.4	8,663
||||||||+ mscorlib.ni!RemotingServices.GetOrCreateIdentity	11.2	8,569
|||||||||+ mscorlib.ni!IdentityHolder.FindOrCreateServerIdentity	11.2	8,552
||||||||||+ mscorlib.ni![COLD] System.Runtime.Remoting.IdentityHolder.SetIdentity(System.Runtime.Remoting.Identity, System.String, System.Runtime.Remoting.DuplicateIdentityOption)	10.6	8,047
|||||||||||+ clr!JIT_IsInstanceOfClass	9.8	7,440

Most of the time spent in RealProxy comes from the xunit test runner itself:

Name	Inc %	Inc
mscorlib.ni!RealProxy.PrivateInvoke	25.1	19,157
+ clr!CTPMethodTable__CallTargetHelper3	25.1	19,156
|+ clr!CallTargetWorker2	25.1	19,156
| + clr!??TransparentProxyStubWorker	25.1	19,156
|  + clr!TransparentProxyStub_CrossContext	25.1	19,156
|   + xunit.runner.utility.net452!OptimizedRemoteMessageSink.OnMessage	15.3	11,674

Can xunit switch from using RealProxy to something far more efficient?

Repro steps

git clone https://github.com/microsoft/CsWin32
cd CsWin32
git checkout 8c6685a6dacde60ec90b1699bb0fdf9d12207d50
Microsoft.Windows.CsWin32.sln

Run the InterestingAPIs class of tests in the net472 TFM, while collecting an ETL trace (I used PerfView)

Resulting ETL trace (use PerfView or VS to analyze).

[bradwilson]

App Domains (and thus proxies) are completely removed from v3, so there's no work to be done here AFAICT.