You can time any computation. So they all have that side effect.
Also, from Javascript you can execute tons of C++ code (e.g. via DOM manipulation). There's no way all of that native code can be guaranteed to run with consistent timing across platforms.
Computations that call into native APIs can be put in the "has observable side effects" category (but in more fine grained treatment, some could have more specific handling).
function computation() { ... }
before = performance.now();
computation();
t = performance.now() - before;