There are some previous considerations to this message:
First, your difference of ~3.4 is around ~2 for me. That’s because you’re either using the MonoGame debug DLL, or you’re compiling your speed test project with Debug, or you’re running your project with F5 (instead of Ctrl+F5), or a combination of those. When testing speeds it’s better to test the “really release mode” (that’s MG release, release compilation, and Ctrl+F5)
Second, DateTime is not an accurate way to measure short amounts of time. It’s better to use Stopwatch.
That said, the reason it’s slower is that the Vector2.Distance call is not inlined, so a call is made. This is always slower than an inlined code execution. When in doubt about this kind of things, launch ILSpy and have a look at the generated code.
Workarounds? You could mark the Vector2.Distance in monogame source code as Aggressive Inlining, but I’m not sure if it’ll work. I’m not an expert but I think that the JIT plays a big role in inlining, so, being the code in another assembly it’s possible that the Vector2.Distance is inlined depending on the JIT. You may get it inlined in net core 3, but not on a previous version.