Hi,
please check your ECC job log details.
Even check SM50 at ECC side during your load.
As i know, During sequential read from table might be impacting your load performance.
so please check your source table and create index on it.
rather than 3.x please convert your data flow into 7.x and use latest methods to load data and speed up dso load and activation steps by using Tx - RSODSO_SETTINGS.
Thanks.