I decided to pay a premium and buy the official Nothing charger from the official website as the OnePlus charger I had was incompatible with the device. Another reason I was convinced to pay almost double the price and buy the Nothing charger was the user feedback in this thread.
Nothing Phone 2a always charges rapidly as advertised, I have never had any problem since then.
Here are a few samples, all tests were executed with screen always off and WiFi always on:
48% - 98% : 35 minutes and 33 seconds
51% - 98% : 35 minutes and 50 seconds
12% - 99% : 50 minutes and 21 seconds
37% - 95% : 33 minutes and 16 seconds
10% - 22% : 4 minutes and 28 seconds
10% - 35% : 11 minutes and 11 seconds
10% - 56% : 21 minutes and 32 seconds
10% - 76% : 31 minutes and 34 seconds
If you can afford to buy the official Nothing charger then I would highly recommend it as it is consistent