Kopia does not (yet) utilize any parallelism during restore (unlike upload which is heavily parallelized) . This means that with a high-latency store (like OneDrive) you may see long restore times, in particular when you have tons of small files. Parallel restore is coming in future versions, for now if you need to restore a lot of small files quickly. syncing the entire repository locally and restoring from there is the recommended approach.
I’ll prioritize pipelined restores for 0.8.