Is compact slow with an almost full disk?

I let my backup drive get 99% full without having done a compact in a while. I started a compact on my system drive archive. My system drive is a 500MB SSD and the archive was occupying about 1.3 TB. It ran for 50 hours without finishing. I figured it might be slow due to low space on the drive, so I stopped it, cleaned up about 300 MB of space, then restarted the compact. (At the time I stopped it, the archive was still about the same size.) It finished literally 5 minutes later, having reduced the archive to 700 MB. Does this slow behavior, followed by rapid finish once there was space, sound like something you would expect?
Thanks,
Mike

It's hard to say exactly what was going on, but I wouldn't say this behavior is surprising.

When a volume fills up, it becomes harder and harder for the filesystem to locate free clusters needed while writing, and files become increasingly fragmented. If the volume is really really full, this slowdown can be excruciating; like 1,000 or more times slower than normal. In other words, an operation that would normally have finished in a 1/10 of a second could take a minute or more.

The first compact after a capture performs garbage collection, which results in rewriting several quanta index files. If this was happening on a nearly full drive, I can imagine abysmal performance.

Your clean up of 300 MB was probably enough to save the volume from fragmentation purgatory, and as soon as the compact was stopped it would have deleted its own temporary index files, freeing more space, and probably restoring much of the filesystem's typical performance.