You would be surprised. It ultimately regresses to "thread per request but with extra steps". I remember truly atrocious codebases that were spamming task.Result everywhere and yet there were performing tolerably even back on .NET Framework 4.6.1. The performance has (and often literally) improved ten-fold since then, with threadpool being rewritten, its hill-climbing algorithm receiving further tuning and it getting proactive blocked workers detection that can inject threads immediately without going through hill-climbing.

Badly written threaded code will have the same problem, unfortunately