Adding a mismatched RAM stick vs buying a matched kit: Flex mode works but costs 3-5% performance

Adding a single 16GB stick to your existing 2x8GB setup triggers flex mode — the first 16GB runs dual-channel, the last 8GB runs single-channel. The performance hit is small (3-5% in CPU-heavy tasks), so you probably won’t notice it in games or daily use. If you want full dual-channel, sell your old kit and buy a 2x16GB set instead.