Will a USB passthrough mousepad slow down my mouse clicks?

No, not in any way you’d ever notice with a wired mouse. USB is a digital protocol with error correction, and a good-quality passthrough hub adds microseconds of latency — far below human perception. Your mouse’s polling rate is already slower than USB 2.0’s bandwidth. Just avoid cheap, unshielded hubs; reputable brands like Razer or Corsair are fine.