\[ \left \{x'(t)=a y(t),y'(t)=b x(t)\right \} \] ✓ Mathematica : cpu = 0.0416904 (sec), leaf count = 158
\[\left \{\left \{x(t)\to \frac {e^{-\sqrt {a} \sqrt {b} t} \left (\sqrt {b} c_1 \left (e^{2 \sqrt {a} \sqrt {b} t}+1\right )+\sqrt {a} c_2 \left (e^{2 \sqrt {a} \sqrt {b} t}-1\right )\right )}{2 \sqrt {b}},y(t)\to \frac {e^{-\sqrt {a} \sqrt {b} t} \left (\sqrt {b} c_1 \left (e^{2 \sqrt {a} \sqrt {b} t}-1\right )+\sqrt {a} c_2 \left (e^{2 \sqrt {a} \sqrt {b} t}+1\right )\right )}{2 \sqrt {a}}\right \}\right \}\]
✓ Maple : cpu = 0.059 (sec), leaf count = 65
\[ \left \{ \left \{ x \left ( t \right ) ={\it \_C1}\,{{\rm e}^{\sqrt {a}\sqrt {b}t}}+{\it \_C2}\,{{\rm e}^{-\sqrt {a}\sqrt {b}t}},y \left ( t \right ) =-{1\sqrt {b} \left ( {\it \_C2}\,{{\rm e}^{-\sqrt {a}\sqrt {b}t}}-{\it \_C1}\,{{\rm e}^{\sqrt {a}\sqrt {b}t}} \right ) {\frac {1}{\sqrt {a}}}} \right \} \right \} \]