Fast then very slow

Experience formula: \begin{cases} \frac{ x^3 \left( 24 + \left\lfloor \frac{x+1}{3} \right\rfloor \right) }{50}, & \text{if } x \leq 15 \\ \frac{ x^3 \left( 14 + x \right) }{50}, & \text{if } 15 < x \leq 35 \\ \frac{ x^3 \left( 32 + \left\lfloor \frac{x}{2} \right\rfloor \right ) }{50}, & \text{if } x > 35 \\ \end{cases}