In case you haven't done so, if you look in the Bitcoin advanced settings, and go optimizations (I believe), then down a bit to "cache size"... By default it's set to 450mb but, depending on how ram you have, you can raise the cache size limit. As an example, 1024mb is 1gb of ram. If you're using an 8gb raspberry pi, I'd say put it up at high as 6gbs to keep your system from becoming unstable. By doing this, you can save yourself some time. You can change it back to default, over your syncing is complete.