
Use your strong hand and begin to insert a cartridge into the magazine while simultaneously pushing down on the spring with the thumb of your weak hand.
Insert the cartridge fully into the magazine until it seats properly. Once you are done, continue to add more cartridges.
It may seem odd at first doing this but I guarantee it will save you time, and “wear and tear” on your thumbs. Being able to use your wrist and forearm muscles to hold the magazine gives you greater control of it and therefore the ability to seat the cartridges more quickly. Try this a half dozen times until you get the hang of it.