Calling powershell.exe is extremely slow
Powershell loads and JIT compiles tons, and tons, and tons of stuff. This takes a lot of excessive CPU cycles and disk I/O.
Try using ngen.exe (Native Image Generator) to precompile the assemblies that Powershell loads during startup. Just might improve your startup time.
$FrameworkDir=[Runtime.InteropServices.RuntimeEnvironment]::GetRuntimeDirectory()
$NGENPath = Join-Path $FrameworkDir 'ngen.exe'
[AppDomain]::CurrentDomain.GetAssemblies() |
Select-Object -ExpandProperty Location |
ForEach-Object { & $NGENPath """$_""" }
Read about ngen here.