Hi, you are not the only one! I am also facing the same problem. My charger takes around 5 hours to charge my iphone 12 from 20 percent to 85. I think its a waste of money, best is to go to the apple reseller and see if you can show them how slow it charges and hopefully they will take it back. For me, they gave me a new one but i had the same problem, so i guess a batch of MagSafe chargers could be faulty.
My reseller here in Pakistan was good to change it. However, they said they have sold hundreds of magsafe but no one else has complained.
its also better not to use MagSafe as it heats up your phone too and fast chargers deteriorate the life span of the battery over time. I would suggest use the brick that came with your iphone. I use the 5 w brick that came with my XR and it take about 2 hours 30 m to fully charge my 12.