How should I swap memory sticks in an HPE DL380 Gen9 to avoid performance issues?

Swap sticks in pairs, not one by one. The DL380 Gen9 uses quad-channel memory per CPU, and mixing unmatched sticks forces lower performance. Pull two sticks from the same channel pair (e.g., slots A1 and A2 for CPU1) and replace them with matched ones. For full speed, use 2R x4 16GB or 32GB DIMMs, all same spec.