✓ Mathematica : cpu = 3.17879 (sec), leaf count = 1
DSolve[Derivative[1][y][x]^2*(-1 + a^2*Derivative[1][y][x]^2) - 2*a^2*y[x]*Derivative[1][y][x]^2*Derivative[2][y][x] + (-b^2 + a^2*y[x]^2)*Derivative[2][y][x]^2 == 0,y[x],x]
✓ Maple : cpu = 2.641 (sec), leaf count = 162
dsolve((a^2*y(x)^2-b^2)*diff(diff(y(x),x),x)^2-2*a^2*y(x)*diff(y(x),x)^2*diff(diff(y(x),x),x)+(a^2*diff(y(x),x)^2-1)*diff(y(x),x)^2=0,y(x))