My iPhone Mobile 12 Pro Max is not charging fast

Fast charging takes place up to 50%. See this: https://support.apple.com/en-us/HT208137. It’s called fast charging because it sacrifices some charging speed in the last 50% in order to charge the first 50% as fast as possible. That way, within 30 minutes or so you have a phone charged enough to be able to use it without worrying about running out of charge quickly. From my experience, the time it takes to charge from 10% to 100% it’s about two hours.

What charger are you using?

50% charge in thirty minutes and then it slows.

Full charge could take an hour and a half.

How fast or slow is it charging?

And how fast did you anticipate it would charge?