Mismatched RAM capacity runs slower.

Yes, there’s a performance penalty. A 4GB soldered + 8GB replaceable setup runs in “asymmetric dual-channel” mode, where only the first 4GB of each stick works in dual-channel, and the remaining 4GB of the 8GB stick runs in single-channel. In practice, you lose about 10-20% memory bandwidth on tasks that hit that extra single-channel region.

The penalty isn’t huge for normal web browsing or office work, but it matters for gaming, video editing, or anything that pushes memory. If you can, get a replacement stick that matches the soldered capacity exactly. If not, just know you’re leaving some speed on the table.

Worst of all: some laptops don’t even let you add RAM, so check your manual before buying anything.