dataflow incremental refresh slower then full refresh

I have a dataflow with 3 tables. 2 of them have the incremental refresh applied.  

- the dataflow takes about 1:57min to complete with incremental mode

- when incremental refresh is turn off it takes about 48min

That's a problem cos I expected incremental would be faster.

Let's take one of these incrementaly refreshed table to examin closely.

- it is configured to refreshed last 45 days, although 48 partitions are created

- the refresh starts at 13:08

- 32 out of 48 partitions are refreshed in parallel starting at 13:08

- rest of te partitions waiting in queue. The last two ones begin refresh at 14:09

here is a screenshot from dataflow refresh logs. (not all the columns and partition visible)

questions:

1) why not all of the partitions are refreshed in parallel? 

2) what can I do to be able to refresh all in parallel? I guess this is a problem why the incremental refresh takes so long. The partition which is refreshed at 14:09 need 37min to complete.