Warning: Performance degradation of data upload possible; see long text

What causes this issue?

Not all the fields are marked for 'Transfer' for the data upload. As a
result there might be a performance degradation since the upload program
will have to read the data field by field.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Select all the fields for data transfer if you choose ASCII upload for
better performance.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.