is core Mutex lock "fast"?

[ludo]

> However, I think this is all moot, druntime is the same as 
> Tango.

Moot you mean debatable? Or irrelevant :) Thanks to your 
explanations, I understand now that the dev tried to imitate a 
Tango feature with very old D1 code. This is 2005/2009 code

And as pointed out by IGotD-, better not to mess around with 
synch / reinvent the wheel. I will trash this class. Still 
learning a lot, thank you guys.