Before making any changes to the current volume, we need to know it's current state of performance. Before the Migration, as you mentioned it was FAS2520, so was it running on hybrid aggregate (flash_pool), or regular aggregate with SAS disks or Capacity disks ? If you have a OCUM , then you could have a performance stats for last 3 to 6 months, to get an avg latency for the volume (which stored the video files).
Since, it has been migrated, is the system (Filer), has similar capacity/performance levels ? You could do QoS from CLI to understand, what's the latency like for the given volume, is there any other volume that is probably eating up most IOS from that aggregate ? Unless, there is a data to analyse it will be difficult to pinpoint the cause.
If it's a AFF300 (running ONTAP), then I can help you with few commands to get the performance data from the current volume, if it's a storagegrid, then I suggest raising a ticket with NetApp Support and work with them, they will be able to guide you on the logs they will need to troubleshoot this issue.
I hope this helps.