Printing an unevaluated Sum in Add can be unexpectedly slow

[ethankward]

>>> from sympy import *
>>> from sympy.abc import q, z, n
>>> f1 = lambda z, q: -log(1 - q)
>>> f2 = lambda z, q: log(q) * Sum(q**(n + z) / (1 - q**(n + z)), [n, 0, oo])
>>> f1(z, q)
-log(1 - q)
>>> f2(z, q)
log(q)*Sum(q**(n + z)/(1 - q**(n + z)), (n, 0, oo))
>>> f1(z, q) + f2(z, q)
log(q)*Sum(q**(n + z)/(1 - q**(n + z)), (n, 0, oo)) - log(1 - q)
>>> f1(3, 4)
-log(3) - I*pi
>>> f2(3, 4)
log(4)*Sum(4**(n + 3)/(1 - 4**(n + 3)), (n, 0, oo))
>>> f1(3, 4) + f2(3, 4)
log(4)*Sum(4**(n + 3)/(1 - 4**(n + 3)), (n, 0, oo)) - log(3) - I*pi

The last line takes about 30 seconds.

[asmeurer]

Likely just a instance of #10800