Synology HA active-passive performance and failover

Decouple your Synology HA setup, RMA one of these units with "Your HA does not work!", and get VMware or similar vSAN to enjoy local disk performance, and reliability. Whatever you'll do you won't squeeze any reasonable IOPS, and transparent failover time out of your current setup.

P.S. You might try to use IBM SVC (now IBM Spectrum Virtualize) or already referenced StarWind to fix broken HA but that's an overkill IMHO.

https://www-03.ibm.com/systems/storage/software/virtualization/svc/

Unfortunately, Synology HA feature does not support active-active mode. If you want to speed things up you might do RAID0 but it’s quite dangerous and will completely load your networks (especially on sequential workloads) while processing backups and large files on a file share. There is an option to use some 3rd party software that will treat your Synology boxes like regular SAN or NAS and do the active-active mechanics on top. In order to do this, you will need at least two servers (but since you’ve mentioned a cluster, I assume you have those). Another good thing about this approach is that failover time will be significantly reduced either.

Here is a great example based on similar DiskStation you have that might give you all the answers you need https://www.starwindsoftware.com/blog/synology-diskstation-ds916-turbocharged-with-starwind-virtual-san

Hope this helps.