I haven't actually timed the backup process. But I suspect it probably is a little bit slower because Lightroom is storing the catalog in a compressed zip file. Any might be taking a little bit longer to do the compression.
Since my backup with large catalog (2.6GB) results in LR6 freezing, I could see that it backs up the catalog in the normal way, and then compresses it and deletes the uncompressed version. When my LR6 freezes, it leaves me with both the uncompressed and compressed versions.
So I presume that would take longer.
Are you guys doing the backup to a location over a network? I put a suggestion in to feedback.photoshop.com about making this zipping backup go faster when writing to slow media (like a network)...see it here:
No, I am not backing up to a network location. Internal HD.