It would be more user-friendly if the currently edited word (the one containing the caret) is excluded from the real-time spell checking and, if incorrect, is not underlined until the caret leaves the word boundary. Otherwise currently edited word is marked as incorrect while its typing is still in progress.

Changes in a word can immediately mark it as correct (i.e. to remove the red underline), but cannot mark it as incorrect - it's marked as incorrect only after the caret leaves the word.