\[ \boxed { {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) -\tan \left ( xy \left ( x \right ) \right ) =0} \]
Mathematica: cpu = 40.401130 (sec), leaf count = 18 \[ \text {DSolve}\left [y'(x)-\tan (x y(x))=0,y(x),x\right ] \]
Maple: cpu = 0.297 (sec), leaf count = 44 \[ \left \{ y \left ( x \right ) =-i{\it RootOf} \left ( -{\it Erf} \left ( { \frac { \left ( -x+{\it \_Z} \right ) \sqrt {2}}{2}} \right ) \sqrt {\pi }-{\it Erf} \left ( {\frac {\sqrt {2} \left ( x+{\it \_Z} \right ) }{2}} \right ) \sqrt {\pi }+\sqrt {2}{\it \_C1} \right ) \right \} \]