They'll run at the slower speed. That's fine.

Yes, you can mix them. The motherboard will automatically downclock both sticks to the speed of the slowest one (DDR4-2400). No BIOS tweaking required for basic operation.

The performance hit is real but usually not noticeable in everyday use. You lose about 25% of the bandwidth on the faster stick, but unless you’re doing memory-intensive tasks like video editing or heavy gaming, you probably won’t feel it.

One catch: mixing different kits can cause stability issues, especially if timings are far apart. It usually works, but it’s not guaranteed. Test with MemTest86 for a few passes if you’re keeping it long-term.

If you already have the 2400 stick, go for it. If you’re buying new, just get matching 3200 sticks. The price difference is negligible these days.