Prior to SQL Server 2014, SQL Server used a Write Ahead Log (WAL) mechanism which means that changes are written to the transaction log before they are committed. The major advantage with this mechanism is that there will be no data loss . However, since IO is the slowest component, there will be a high IO contention for large writes especially if you have a low performing disk configuration for log files.
Delay Transaction Durability is introduced in SQL Server 2014 to enhance write performances, read this article for more details.
