One long transaction or multiple short transactions?

We have a system which is constantly importing flat file data feeds into
normalized tables in a DB warehouse over 10-20 connections. Each data feed
row results in a single transaction of multiple single row writes to
multiple normalized tables.

The more columns in the feed row, the more write operations, longer the
transaction.

Operators are noticing that splitting a single feed of say - 100 columns -
into two consecutive feeds of 50 columns improves performance dramatically.
I am wondering whether the multi-threaded and very busy import environment
causes non-linear performance degradation for longer transactions. Would the
operators be advised to rewrite the feeds to result in more smaller
transactions rather than fewer, longer ones?

Carlo