Mention typical execution time

Author: jussienko

performance/cprofile/README.md

@@ -4,7 +4,7 @@ In this exercise we analyze the performance of heat equation solver with cProfil
 
 The file [heat_main.py](heat_main.py) contains the (very inefficient)
 implementation of the two dimensional heat equation. Use `cProfile` for
-investigating where the time is spent in the program. You can try to profile
-also the more efficient model solution of
-[numpy/heat-equation](../heat-equation/solution/heat-serial.py).
+investigating where the time is spent in the program. Note that the execution 
+time can be between 40 - 60 s depending on your hardware. (You can see also 
+results of the simulation in the *heat_nnn.png* output files).