Oh, I've run into similar issues before with IIS 10.0 Express! It can be tricky when it starts to choke under multiple simultaneous requests. One thing that really helped me was tweaking the application pool settings. Specifically, increasing the Queue Length and adjusting the Rapid-Fail Protection settings made a notable difference.
Also, produce sure your Max Concurrent Requests setting isn't too low. This can drastically affect how well your server handles load.
On another note, if you're developing something that will face high traffic in production, consider testing it as well on a full version of IIS if possible. The Express version has some limitations which might not give you a complete picture of how things will behave in a live environment.
Hope this helps!
