Yeah, it's the preallocation thing you'll see in Steam when updating. I've noticed this quite often with UE based gamesThe update progress of KartKraft indeed is pretty slow. It also needs the full amount of the whole games disk space for it on the hard drive to be free in addition, probably a reason, why it takes so long. Some games do that for some reason.^^
Some games can simply patch themselves on the fly so to say - just download the patch and apply it directly. Other games, a la Wreckfest, seem to need to download the whole game again - absolute pain for those with limited bandwidth. And then there's those that need to be "preallocated". AFAIK this is where the game, or at least significant parts of it, needs to be copied and rewritten while adding the patch, much in the same way the PS4 handes it's updates.