is it possible for you to use the COMMIT option, at least for testing ?
Specifying COMMIT=Y prevents rollback segments from growing inordinately large and improves the performance of large imports, but it is only advisable if the table has uniqueness constraints, so if the import fails and it is restarted, any rows that have already been imported are rejected and the data does not get duplicated.
I would also try using the FEEDBACK parameter, which could help seeing if the import performance decreases gradually.
For example, if you specify FEEDBACK=100, Import should display a dot each time 100 rows have been imported.
It could also be useful specifying a log file (LOG=filename.log), but I guess if you don’t see any error messages on screen, no error is going to be logged either.