How to speed up ToString[#,TeXForm]&?

Try the following code to convert the expression to LaTeX code

f = Expand[(x/2 + y/3 + z/5 + 1/7)^20];str = ToString[f,TeXForm]; // AbsoluteTiming

It took about 1 minute, while sympy.latex only took 0.6 seconds. How can I speed it up?