I have just installed the latest stable updates (Xamarin 3.5.55 on PC, Xamarin.iOS 126.96.36.199 on Mac) and builds in Visual Studio are now taking several times longer than they were.
Using the previous version of Xamarin, builds took a few minutes. Now, they are taking over 20 minutes.
Looking at the build host log, some requests are taking a very long time - e.g. "Request handled in 149140.028ms".