\[ a \sin (y(x))+y''(x)=0 \] ✓ Mathematica : cpu = 0.0821563 (sec), leaf count = 79
\[\left \{\left \{y(x)\to -2 \text {am}\left (\frac {1}{2} \sqrt {(2 a+c_1) (x+c_2){}^2}|\frac {4 a}{2 a+c_1}\right )\right \},\left \{y(x)\to 2 \text {am}\left (\frac {1}{2} \sqrt {(2 a+c_1) (x+c_2){}^2}|\frac {4 a}{2 a+c_1}\right )\right \}\right \}\] ✓ Maple : cpu = 0.379 (sec), leaf count = 49
\[ \left \{ \int ^{y \left ( x \right ) }\!{\frac {1}{\sqrt {2\,a\cos \left ( {\it \_a} \right ) +{\it \_C1}}}}{d{\it \_a}}-x-{\it \_C2}=0,\int ^{y \left ( x \right ) }\!-{\frac {1}{\sqrt {2\,a\cos \left ( {\it \_a} \right ) +{\it \_C1}}}}{d{\it \_a}}-x-{\it \_C2}=0 \right \} \]