How many services are you pushing? How many instances of each service? What OS? Are you using a web adapter? Https? IWA or GIS Token? Image Service cached? There are about 2 dozen architecture factors that seriously impact performance, but, all that aside, with just two "cores", I'm surprised you can even get ArcGIS Server to start. On a VM, I had to push it to 16 cores to see any stability. What type if hard drive? Is it a shared VHD where dozens of other Vm's are using the same HDD? You should choose clustering only as a last resort, and I really emphasize "last resort" (e.g ESRI tech support told you there is no other way your site is going to work unless in a cluster). Clustering means you have a shared config store, so every other round-robin request for your image service has to go over the network via SMB to the config store. Get rid of the cluster, and add the RAM/CPU resources to the single instance. Or....run two sites, one on each VM, one for vector data, and a dedicated one for image. But by all means, avoid clustering.
