TIL stack exchange is built on ASP.NET
And there I thought the platform had serious scalability issues

.NET has very good performance and netcore even more.

If you really does hit a ceiling its either database, your own code, some other bottle neck or you should scale to multiple servers anyway :)

Then ofcause. Stack Exchange is probably among the best in performance optimizations on .net and are also great in that many of their tools are open source and they blog feverishly about how they work to achieve this performance.