Back-up of the domain - Timing of the script

Hi,

This is with regards to the domain backup that we perform and use the below commands in the script to execute the same.

tar cvf ${TarName} $DomainPath >>${LogFile}
compress ${TarName} >> ${LogFile}

So, basically tar the domain and than compress the same. The issue that we see is with regards to the time it takes and is not consistent every other day. Some days it take around 18min while on other days the same process completes within 8minutes. Any insights into what could be the reason for this time difference.