The live view was fetching a page title on every frame.
Every command notes where a browser is, so a session can be saved where it was, and it did that by asking for each tab's whole description - title included, which costs a round trip per tab while the url does not. Measured A-B-A on one bench, median of twenty frame requests: 8.3 ms with urls only, 29.5 ms with the title, 6.8 ms with urls again.