We had the same issue in our installation. But, we knew the reason for it. When the TD14 was upgraded, the BAR was still not upgraded and it was using a public network, so the speed was slow. After that TD GSC upgraded the BAR and moved it closer to the TD box and started using provate network, then our backups were fast, in fact faster than TD13.10. Pls check if that is a reason for you also.
Assuming this is not the SLES 11 kernel issue noted, the backup design hasn't been altered, and the system performance/resource availability is the same.
Also assuming we are talking about an ARC based backup solution
One thing with major upgrades is the Workload Management rules get reset on the backup account sessions back down to the default of 4 sessions, you can normally tell by looking at an ARC log near the begining
ARC HAS REQUESTED 25 SESSIONS, TASM HAS GRANTED IT 4 SESSIONS
Session count per stream should have been calculated for balanced backups (if possible) factoring in node type(s), AMP count per node, and backup strategy & infrastructure.