With spell check on, words will be underlined as misspelled as you type, so nearly every word you’re in the middle of typing will get a red underline. This - for me - ruins the polished feel I’m hoping Obsidian can achieve - to have this flashing red underline on every character I type.
Wait until there is a space or punctuation mark before deciding if the word is misspelled.
Typora, for example, does this. However, I will note that the way Obsidian does this now seems more common than not.
Turn off spellcheck.