\[ a y'(x)^2+b \sin (y(x))+y''(x)=0 \] ✓ Mathematica : cpu = 3.98017 (sec), leaf count = 146
\[\left \{\left \{y(x)\to \text {InverseFunction}\left [\int _1^{\text {$\#$1}}-\frac {\sqrt {4 a^2+1}}{\sqrt {4 e^{-2 a K[1]} c_1 a^2-4 b \sin (K[1]) a+e^{-2 a K[1]} c_1+2 b \cos (K[1])}}dK[1]\& \right ][x+c_2]\right \},\left \{y(x)\to \text {InverseFunction}\left [\int _1^{\text {$\#$1}}\frac {\sqrt {4 a^2+1}}{\sqrt {4 e^{-2 a K[2]} c_1 a^2-4 b \sin (K[2]) a+e^{-2 a K[2]} c_1+2 b \cos (K[2])}}dK[2]\& \right ][x+c_2]\right \}\right \}\] ✓ Maple : cpu = 0.402 (sec), leaf count = 115
\[ \left \{ \int ^{y \left ( x \right ) }\!{(-4\,{a}^{2}-1){\frac {1}{\sqrt {16\,{\it \_C1}\, \left ( {a}^{2}+1/4 \right ) ^{2}{{\rm e}^{-2\,{\it \_a}\,a}}-16\, \left ( {a}^{2}+1/4 \right ) b \left ( a\sin \left ( {\it \_a} \right ) -1/2\,\cos \left ( {\it \_a} \right ) \right ) }}}}{d{\it \_a}}-x-{\it \_C2}=0,\int ^{y \left ( x \right ) }\!{(4\,{a}^{2}+1){\frac {1}{\sqrt {16\,{\it \_C1}\, \left ( {a}^{2}+1/4 \right ) ^{2}{{\rm e}^{-2\,{\it \_a}\,a}}-16\, \left ( {a}^{2}+1/4 \right ) b \left ( a\sin \left ( {\it \_a} \right ) -1/2\,\cos \left ( {\it \_a} \right ) \right ) }}}}{d{\it \_a}}-x-{\it \_C2}=0 \right \} \]