Both sticks run at the slower speed.

Yes, but your RAM will clock down to the lowest common denominator. If you have 2666MHz soldered and add a 3200MHz stick, both run at 2666MHz. You can’t force the faster speed — the memory controller just won’t allow it.

Is it usually fine? Depends on what you’re doing. For normal desktop work, light gaming, or web browsing, you won’t notice the difference. For memory-bound tasks—video editing, large compiles, heavy simulation—you’re leaving some performance on the table. You might also have to deal with looser timings if the sticks are mismatched.

One gotcha: some laptops or prebuilts are picky. Mixing a soldered stick with a replaceable one can sometimes cause stability issues. Check your motherboard’s QVL or just test it with MemTest86 for a few hours.

If you’re buying new, just match speeds. If you already have a soldered stick, buy the same speed—don’t spend extra on faster RAM you can’t use.