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Because pure python is slow. No offense but why would you want to do this? Symbolic calculations are usually done with pynac (C++ wrapped into Sage) or Maxima (40 year old optimized Lisp wrapped into Sage). If you want something about symbolic differentiation improved you should contribute to those projects.

What do you need that isn't already there? For your example on my computer:

var('x,k')
time diff(sum(k*x^k,k,0,10),x)

gives

100*x^9 + 81*x^8 + 64*x^7 + 49*x^6 + 36*x^5 + 25*x^4 + 16*x^3 + 9*x^2 + 4*x + 1
Time: CPU 0.02 s, Wall: 0.04 s