Hi! Which annotation interface are you using? Are you using the relations UI? If so, that can currently become slower with larger documents (the specifics also depend on your browser and machine). I definitely want to work on improving the performance here – the tricky part is that the interface is quite complex so we might need a better way to represent the tokens and spans (which technically all have to be able to be connected to any other token or span).
If you're using any other text-based interface like ner_manual, there shouldn't be any performance differences, even if you're using huge texts, since they render plain text and rely on the browser's native highlighting. If you do end up having problems here, that'd definitely be interesting to investigate, because it might indicate that something else is going on here!