It really depends. If the only thing you care about rendering is US ASCII with a monospace bitmapped font, that's not too hard.
However, a general purpose text renderer will have to handle things like directionality (left to right versus right to left text orientation), wide/half text rendering (for CJK characters), combining characters (for some European, South East Asian languages, and zalgo), line break rules (handling characters like soft hyphens and non breaking spaces), and so on.
On the font rendering side of things, there's kerning (some letters are closer to one another), font ligatures (certain fonts handle things like fi ffi fl ffl differently, not to mention things like hasklig), antialiasing, sub pixel rendering, and the fact that fonts are actually programs running on a stack based virtual machine [0].
That's also ignoring more advanced text rendering features like ruby characters and vertical text rendering. Or that apparently Turkish has different small caps than English ones [1].
In practice, it's probably not the text rendering that's the issue, it's probably just features that are implemented inefficiently.
[0] https://developer.apple.com/fonts/TrueType-Reference-Manual/...
[1] https://docs.microsoft.com/en-us/typography/opentype/spec/tt...