I seem to recall reading somewhere that 'content-visibility' has a performance penalty, because the browser effectively sticks intersection observers on the elements marked with content-visibility:auto; so that setting content-visibility:auto on, say, thousands of table rows delays page render. But now I can't find where I saw that.

That makes sense. Ideally in that scenario you don't set it at table row level, rather at a higher container level. In this case a container wrapping things far down the page. Your markup structure of course has to support this.