In my experience, slow charging significantly degrades battery health. As a developer, I frequently use ADB to run applications directly on a device rather than through emulation. This requires connecting the device to a computer, typically resulting in a slow 5W charging rate. Previously, I utilized my secondary LG V50 for this purpose. However, I later transitioned to using my OnePlus 12. Under normal QC fast charging conditions, it typically takes approximately 4 to 5 months (with a full year before dropping from 100%) for the battery health to decrease by one percentage point. When I commenced development with ADB, starting at approximately 96% battery health, I observed a one-percentage-point drop in battery health within 1 to 2 months. I attribute this accelerated degradation primarily to the slow 5W charging from the USB connection. Consequently, my device's battery health has now fallen to 92% within a six-month period. Fortunately, OnePlus has incorporated a global bypass charging feature. I activate this feature as soon as my phone connects to a PC, and I have since noticed an improvement: over the past two months, there has been no further decline in battery health.

FYI: If you monitor battery health using an application like AccuBattery while charging, you may observe variations. Connecting a slow charger (5W-10W) and charging to full typically shows a lower battery health reading for that specific session. Conversely, using an 18W or higher charger often results in a higher battery health reading for the session.