smichr/sympy — comment on commit a00f5e8c5ffec22f2a15acc2b105478cca9a6619
File:  line 0
By asmeurer:

So I think that eventually subs should move away from doing one substitution at at time to doing them all at once, for performance reasons (otherwise you have to recurse the expression tree for every substitution). How do you think this change would fit into a change like that?