My computer is Mac Pro M1, 2020.
General Comments: for expand, SymEngine is nearly as good as Mathematica, Symbolics‘s “expand” function still needs to be greatly improved. substitute are similar for the three CAS in performance.
f=(x+big(2)y)^6000|>expand; # 64.2s f=(x+big(2)y)^100000|>expand; # looks infinity
f=(x+2y)^6000|>expand; #0.5s f=(x+2y)^100000|>expand; # 10.6s
Mathematica takes 0.01s and 3.1s respectively.
Symbolics takes 0.9s
SymEngine takes 0.4s
Mathematica takes 0.06s