The faster RAM downclocks to match the slower stick.

Yes, there’s a penalty. The system will run both sticks at the lower speed. If your soldered RAM is DDR4-3200 and you add a DDR4-2933, both will drop to 2933. You just paid for speed you won’t use.

That’s the main hit: you lose the extra bandwidth on the soldered stick. In real-world tasks you probably won’t notice unless you’re doing memory-bandwidth-heavy work like video editing or certain games.

Also, you might lose dual-channel operation if the capacities or ranks don’t match. Most laptops with soldered + socketed RAM run in “flex mode” – dual channel for the shared portion, single for the rest. That’s usually fine but not ideal.

If you can find a stick that matches the soldered speed exactly, do that. If not, the penalty is small enough that adding more capacity is usually