Evidently, when Time Machine encounters a file that’s open and being written to (e.g. a Safari download), it sits and watches the file, periodically copying the new data to the backup. As opposed to going off and copying everything else first, and then coming back to the open file.
This has the lovely effect that 1.4 TiB of my backup is wedged behind a couple of GiB of remaining download, ensuring the overall backup process takes hours longer than necessary. Well done Time Machine, you suck.