\[ \boxed { \left ( y \left ( x \right ) \right ) ^{2}{\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) +y \left ( x \right ) \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{2}-ax-b=0} \]
Mathematica: cpu = 0.488562 (sec), leaf count = 32 \[ \text {DSolve}\left [-a x-b+y(x)^2 y''(x)+y(x) y'(x)^2=0,y(x),x\right ] \]
Maple: cpu = 2.012 (sec), leaf count = 172 \[ \left \{ {\frac {b\ln \left ( ax+b \right ) }{a}}-\int ^{{\frac {y \left ( x \right ) }{ax+b}}}\!{\frac {1}{2\,\sqrt {3}{{\it \_g}}^{3}{a} ^{2}-2\,\sqrt {3}} \left ( 3\,{b}^{2}{{\it \_g}}^{2}\sqrt [3]{-{\frac { a}{{{\it \_g}}^{3}{b}^{3}}}}\tan \left ( {\it RootOf} \left ( 6\,{b}^{2} \int \!{\frac {{{\it \_g}}^{2}}{{{\it \_g}}^{3}{a}^{2}-1} \left ( -{ \frac {a}{{{\it \_g}}^{3}{b}^{3}}} \right ) ^{2/3}}\,{\rm d}{\it \_g}-2 \,{\it \_Z}\,\sqrt {3}+\ln \left ( {\frac { \left ( \tan \left ( {\it \_Z} \right ) \right ) ^{2}+1}{3+2\,\sqrt {3}\tan \left ( {\it \_Z} \right ) + \left ( \tan \left ( {\it \_Z} \right ) \right ) ^{2}}} \right ) +6\,{\it \_C1} \right ) \right ) +{b}^{2}{{\it \_g}}^{2}\sqrt [3]{-{\frac {a}{{{\it \_g}}^{3}{b}^{3}}}}\sqrt {3}-2\,{{\it \_g}}^{2}a b\sqrt {3} \right ) }{d{\it \_g}}-{\it \_C2}=0 \right \} \]