\[ y'(x)-f(a x+b y(x))=0 \] ✓ Mathematica : cpu = 0.211784 (sec), leaf count = 248
DSolve[-f[a*x + b*y[x]] + Derivative[1][y][x] == 0,y[x],x]
\[\text {Solve}\left [\int _1^{y(x)}-\frac {f(a x+b K[2]) \int _1^x\left (\frac {b^2 f'(a K[1]+b K[2])}{a+b f(a K[1]+b K[2])}-\frac {b^3 f(a K[1]+b K[2]) f'(a K[1]+b K[2])}{(a+b f(a K[1]+b K[2]))^2}\right )dK[1] b+b+a \int _1^x\left (\frac {b^2 f'(a K[1]+b K[2])}{a+b f(a K[1]+b K[2])}-\frac {b^3 f(a K[1]+b K[2]) f'(a K[1]+b K[2])}{(a+b f(a K[1]+b K[2]))^2}\right )dK[1]}{a+b f(a x+b K[2])}dK[2]+\int _1^x\frac {b f(a K[1]+b y(x))}{a+b f(a K[1]+b y(x))}dK[1]=c_1,y(x)\right ]\] ✓ Maple : cpu = 0.044 (sec), leaf count = 37
dsolve(diff(y(x),x)-f(a*x+b*y(x)) = 0,y(x))
\[y \left (x \right ) = \frac {\RootOf \left (\left (\int _{}^{\textit {\_Z}}\frac {1}{f \left (\textit {\_a} b \right ) b +a}d \textit {\_a} \right ) b -x +c_{1}\right ) b -a x}{b}\]