About Master Level

The master level only impacts your friend list max ap and your max formation list total. It has no other impact other than the higher it gets the less often you get a full ap recovery. Honestly getting your servants to max level isn't that hard. It's getting their skills maxed that takes the most time. As long as you feel you're making good progress I wouldn't worry about what other people have. This game doesn't have any pvp content so making progress at your own pace is fine.

Starting around level 100 you'll progress much more slowly. Enjoy it while it lasts. ;) I believe I hit level 100 after two months or so, and after that I needed six more months to get to level 125.

You can see in this table that there's a huge leap in additional XP required to raise your level from 100 to 101.
https://grandorder.gamepress.gg/experience-table

Being a high master level with low leveled servants doesn’t hurt you at all so that doesn’t really matter.

It's not a bad thing. Once you reach level 100, the levels slow down a *lot*. That's why it looks like those people have accomplished so much, even though their levels aren't much higher than yours. I'm too lazy to check the actual numbers, but it probably requires as much exp to go from level 100 to 101 as it took you to go from 1 to 80.

However, that means that once you hit level 100, you can't really count on getting a bunch of free AP refills from leveling anymore. So you should consider how you want to spend your remaining refills. Personally, I made a big blunder and got myself to level 113 before I even completed the Orleans singularity, so now it's really hard for me to catch up on the story mode, since I don't have those refills to rely on.