Slow charging was one of the things Apple worked on on the newer systems. Making large improvements in battery design as well as tightening up the charge rate.
Independent of your system & batteries condition
The older MacBook’s battery chemistry is also very different from your newer 15” MacBook Pro’s. This has a big bearing on the charging schedule the charging logic on the logic board follows [https://batteryuniversity.com/index.php/learn/article/charging_lithium_ion_batteries|Charging Lithium-ion]