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