\[ \boxed { {\frac {{\rm d}^{2}}{{\rm d}{x}^{2}}}y \left ( x \right ) -a\sqrt {b \left ( y \left ( x \right ) \right ) ^{2}+ \left ( {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) \right ) ^{2}}=0} \]
Mathematica: cpu = 0.834106 (sec), leaf count = 31 \[ \text {DSolve}\left [y''(x)-a \sqrt {b y(x)^2+y'(x)^2}=0,y(x),x\right ] \]
Maple: cpu = 0.203 (sec), leaf count = 36 \[ \left \{ y \left ( x \right ) ={{\rm e}^{\int \!{\it RootOf} \left ( x- \int ^{{\it \_Z}}\! \left ( -{{\it \_f}}^{2}+a\sqrt {{{\it \_f}}^{2}+b} \right ) ^{-1}{d{\it \_f}}+{\it \_C1} \right ) \,{\rm d}x+{\it \_C2}}} \right \} \]