Yes, it will run at the slower module's speed.

RAM is a “weakest link” situation. The memory controller forces everything to match the slowest stick’s timings and frequency, whether that stick is soldered or in a slot. If your soldered RAM runs at 3200MHz CL22 and you add a 3600MHz CL18 stick, the new one gets downgraded to 3200MHz CL22.

Why it works that way: The system needs all modules to sync up for stability. It can’t run one stick faster than the other. So whatever the slowest stick’s JEDEC profile says, that’s what you get. Overclocking or XMP profiles won’t help if the soldered RAM is fixed.

One thing to check: If the soldered RAM is single-channel and you add a second stick that also runs single-channel, you might still get dual-channel mode (which is a nice performance bump), but the timings and frequency are still dictated by the slower module.

Bottom line: Don’t buy premium fast RAM for a system with soldered memory. You’re paying for speed you won’t get.