Update README.rst

Author: thomasaarholt

README.rst

@@ -11,9 +11,13 @@
 ``adaptive`` is an open-source Python library designed to
 make adaptive parallel function evaluation simple. With ``adaptive`` you
 just supply a function with its bounds, and it will be evaluated at the
-“best” points in parameter space. With just a few lines of code you can
-evaluate functions on a computing cluster, live-plot the data as it
-returns, and fine-tune the adaptive sampling algorithm.
+“best” points in parameter space, rather than unecessarily computing *all* points. 
+With just a few lines of code you can evaluate functions on a computing cluster, 
+live-plot the data as it returns, and fine-tune the adaptive sampling algorithm.
+
+``adaptive`` shines on computations where each evaluation of the function
+takes *at least* ≈100ms. Other approaches for parallisation should be chosen for 
+function calls that are faster than this.
 
 Run the ``adaptive`` example notebook `live on
 Binder <https://mybinder.org/v2/gh/python-adaptive/adaptive/master?filepath=example-notebook.ipynb>`_