In-game performance doesn't directly matter, just wins or losses, in so far as a win where you fed all game and got carried counts the same as a game where you 1v9 hard carried (but of course if you personally do well consistently, you'll win more often).
When you win games, you gain (hidden) mmr and LP, and when you lose you lose mmr and LP. The amount of LP you gain/lose depends on your mmr (and your mmr also determines where you get placed after placements), so if you have a high mmr relative to your rank, you gain a lot of LP and lose very little, while if your mmr is low relative to your rank you'll lose a lot for losses and only gain a little for wins. When your MMR is about 'right' for you rank, it's roughly equal gains and losses of around 19 or 20.
The amount of mmr you gain/lose for wins/losses depends on your mmr relative to your opponents mmr. Usually this doens't matter too much since matchmaking puts people of similar mmrs together (thats what mmr is for really), but it can become a factor when premades are involved, as well as in long-ish queues where matchmaking begins to allow increasingly large mmr differences.
(It's worth noting you can't actually see your mmr anywhere, sites like op.gg just estimate it but it's not necessarily accurate.)