I made an initial backup of multiple paths of my NAS, about 600 GB. Reading from the NAS was the bottleneck. To simplify, I backed up n:\a, n:\b and n:\c (I’m on Windows). After about 2 hours or so the backup was done and I realized I also needed n:\d, so I backed that up as well.
Then, to be complete, I wanted to make a snapshot of all 4 paths, so I started one last backup on all folders at the same time. To my surprise, Restic started to read everything again, so the final snapshot is going to take another 2 hours. Why does it do that? It already has all the information it needs. I think this is a very big disadvantage of combining multiple paths in a single snapshot when datasets are relatively large.
Assuming the paths to backup are identical across the snapshots, as they appear to be in your case, I believe specifying the previous two snapshots as parents would have helped with the initial speed of the third “combined path” snapshot: