Really vague so hard to tell what the issue really is.
No need to change to SSIS - but you may need to change your process a bit depending on...

from my own experience with a few Tabular projects performance will depend mostly on the volume, type of data and cardinality of the data (which also affects compression).

You may be able to get around it on SSRS reports (and eventually powerbi but not sure as Tabular requires Windows Authentication)

1 - Do not grant users access...

using the ACE driver (2010 or 2016) it seems to work fine.
If that isn't what you are using give it a try
https://www.microsoft.com/en-ie/download/details.aspx?id=10910
https://www.microsoft.com/en-us/download/details.aspx?id=50040

Also note that some...

Unless your package is executing a sql statement using linked servers against either A or B with no data flow present then yes it is flowing from A => C...

file seems to be missing a valid value for customerkey - when you preview the data on the source component does it show you values for the customerkey column?
If...

Partitioning will most likely not help here.

where exactly is the contention happening?
And are server specs up to scratch? sometimes performance is due to server either being too...

create one foreach loop for the folders based on a record set created by a script
then within the foreach do another foreach for each file within that folder.

If you can't do explain plans better ask your DBA's to give you that - it is a must for any developer and there is no reason not to be...

In order to allow us to help you we will need the explain plan of the execution of the query.

tables DDL and indexes will also help.

In...

As Phil mentioned IDENTITY and SEQUENCE are the most common and easier to work with .

If not feasible something like this will be the next best thing e.g....