Improve startup time by triggering initialization of additional types on background thread (#18195)

src/System.Management.Automation/engine/InitialSessionState.cs

@@ -44,22 +44,28 @@ namespace System.Management.Automation.Runspaces
 
             // We shouldn't create too many tasks.
 #if !UNIX
-            // Amsi initialize can be a little slow
+            // Amsi initialize can be a little slow.
             Task.Run(() => AmsiUtils.WinScanContent(content: string.Empty, sourceMetadata: string.Empty, warmUp: true));
 #endif
+            // Initialize the types 'Compiler', 'CachedReflectionInfo', and 'ExpressionCache'.
+            // Their type initializers do a lot of reflection operations.
+            // We will access 'Compiler' members when creating the first session state.
+            Task.Run(() => _ = Compiler.DottedLocalsTupleType);
 
             // One other task for other stuff that's faster, but still a little slow.
             Task.Run(() =>
             {
-                // Loading the resources for System.Management.Automation can be expensive, so force that to
-                // happen early on a background thread.
+                // Loading the resources for System.Management.Automation can be expensive,
+                // so force that to happen early on a background thread.
                 _ = RunspaceInit.OutputEncodingDescription;
 
                 // This will init some tables and could load some assemblies.
-                _ = TypeAccelerators.builtinTypeAccelerators;
+                // We will access 'LanguagePrimitives' when binding built-in variables for the Runspace.
+                LanguagePrimitives.GetEnumerator(null);
 
                 // This will init some tables and could load some assemblies.
-                LanguagePrimitives.GetEnumerator(null);
+                // We will access 'TypeAccelerators' when auto-loading the PSReadLine module, which happens last.
+                _ = TypeAccelerators.builtinTypeAccelerators;
             });
         }
     }