\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) -h \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) ,ax+by \left ( x \right ) \right ) =0} \]
Mathematica: cpu = 0.130517 (sec), leaf count = 26 \[ \text {DSolve}\left [y''(x)-h\left (y'(x),a x+b y(x)\right )=0,y(x),x\right ] \]
Maple: cpu = 0.094 (sec), leaf count = 117 \[ \left \{ y \left ( x \right ) ={\it ODESolStruc} \left ( -{\frac {a \left ( \int \!{\it \_b} \left ( {\it \_a} \right ) \,{\rm d}{\it \_a}+{ \it \_C1} \right ) -b{\it \_a}}{b}},[ \left \{ {\frac {\rm d}{{\rm d}{ \it \_a}}}{\it \_b} \left ( {\it \_a} \right ) =-h \left ( -{\frac {a{ \it \_b} \left ( {\it \_a} \right ) -b}{{\it \_b} \left ( {\it \_a} \right ) b}},b{\it \_a} \right ) \left ( {\it \_b} \left ( {\it \_a} \right ) \right ) ^{3} \right \} , \left \{ {\it \_a}={\frac {ax+by \left ( x \right ) }{b}},{\it \_b} \left ( {\it \_a} \right ) ={\frac {b }{a+b{\frac {\rm d}{{\rm d}x}}y \left ( x \right ) }} \right \} , \left \{ x=\int \!{\it \_b} \left ( {\it \_a} \right ) \,{\rm d}{\it \_a }+{\it \_C1},y \left ( x \right ) =-{\frac {a \left ( \int \!{\it \_b} \left ( {\it \_a} \right ) \,{\rm d}{\it \_a}+{\it \_C1} \right ) -b{ \it \_a}}{b}} \right \} ] \right ) \right \} \]