\[ \left (y'(x)^2+1\right ) \left (a x+\tan ^{-1}\left (y'(x)\right )\right )+y'(x)=0 \] ✓ Mathematica : cpu = 1.676 (sec), leaf count = 58
\[\text {Solve}\left [\left \{y(x)=\frac {1}{a \left (K[1]^2+1\right )}+c_1,x=\frac {-K[1]+K[1]^2 \left (-\tan ^{-1}(K[1])\right )-\tan ^{-1}(K[1])}{a \left (K[1]^2+1\right )}\right \},\{y(x),K[1]\}\right ]\] ✓ Maple : cpu = 0.07 (sec), leaf count = 30
\[y \relax (x ) = \int \tan \left (\RootOf \left (a x \left (\tan ^{2}\left (\textit {\_Z} \right )\right )+\left (\tan ^{2}\left (\textit {\_Z} \right )\right ) \textit {\_Z} +a x +\tan \left (\textit {\_Z} \right )+\textit {\_Z} \right )\right )d x +c_{1}\]