The bug

Since Minecraft 1.12 (or a bit before), some of the font rendering was remade, and a minor performance issue was introduced.

In the ChatFormatting class:

public static ChatFormatting getByCode(char c) {
    char lower = Character.toString(c).toLowerCase(Locale.ROOT).charAt(0);

Here, there is a useless conversion from a character to a string, a lowercase string, then a character again. A simple Character.toLowerCase(c) would have been enough.

This issue was even worse before 1.12, where the whole rendered text was being changed to lowercase on each color character, and was causing a ~1% performance drop when rendering a lot of text. Here the impact is minor, but would still be a good improvement to clean it up.