I have installed the latest version 11.3 in a dual boot laptop with Windows 7 and Ubuntu 16.04. The laptop has a quad core i7 CPU capable of reaching 3.9 GHz (with turbo boost).
In both operating systems I have noticed the following: when I open Mathematica the very first execution takes a while to complete. For example, even the trivial 2 + 2 needs about 2 seconds for giving the answer. Then, if I repeat the calculation, or try another one the speed is normal (2 + 2 instantly).
So, why there is such a hesitation during the very first calculation? has anyone else experienced a similar situation? If so, is, there a way to fix that?