If you have slow upload, Duplicati’s own backup to cloud will be very slow and you will not be protected at all until it completes…
But if you first backup to local destination, you at least have full local backup in case you needed it.
The size of incremental chunks depends on how much you change data… YMMV, but my incremental portions are very small…
In any case, setups like this are double-edged sword anyway - you have to decide on what it priority for you - complete local backup and slow sync to cloud or less complete, but in progress cloud backup…
-D