Processing multiple dirs in Backup mode - very ineffective!

I'm using Backup mode (archiver 7z, no compression).
I've added 5 directories to backup to the list on the right, say:
c:\dir1
c:\dir2
c:\dir3
c:\dir4
c:\dir5
The first one is 100Gb, the others - 1Gb each.
The resulted file with backup is "d:\backup.7z"

So, how it works when I first time press "Run"?
1. it creates archive based on "c:\dir1", so after several hours we have "d:\backup.7z" file with size of 100Gb.
2. it adds "c:\dir2" to the existing "d:\backup.7z" archive in 2 steps
1) creates "d:\backup.7z.tmp" based on "d:\backup.7z" - this file has size of 101Gb, and it takes several (twice as much - same drive) hours (!) to add this 1Gb (!) dir into the archive.
2) replaces old "d:\backup.7z" with newer "d:\backup.7z.tmp"
This procedure (copying 100Gb of data within the same physical drive) - is extremly slow!!
3. now step (2) applies to the rest of small directories ("c:\dir3", "c:\dir4", "c:\dir5"
)

So, what we have here.
To backup 104Gb of data toucan has to process 100+101+102+103+104 = 510 Gb of data!
But not just copy them from one drive to another: most of the copying happens within the same drive(!)

So, for the sample above it works at least 9 times slower then it should(!)
E.g. it is 4 full days (more then a halfweek(!)) instead of 10 hours.

Any thoughts?