\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) =-{\frac {x{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) }{f \left ( x \right ) }}+{\frac {y \left ( x \right ) }{f \left ( x \right ) }}=0} \]
Mathematica: cpu = 0.213027 (sec), leaf count = 28 \[ \text {DSolve}\left [y''(x)=\frac {y(x)}{f(x)}-\frac {x y'(x)}{f(x)},y(x),x\right ] \]
Maple: cpu = 0.063 (sec), leaf count = 31 \[ \left \{ y \left ( x \right ) ={\it \_C1}\,\int \!{{\rm e}^{\int \!{ \frac {1}{x} \left ( -2-{\frac {{x}^{2}}{f \left ( x \right ) }} \right ) }\,{\rm d}x}}\,{\rm d}xx+{\it \_C2}\,x \right \} \]