Does Ballooning, Compression, and Swapping happen for your VMs (the .vswp file exists per default when no memory reservation is done)? You can check the Memory Performance Metrics of the Host or the VMs get a good insight.
If one of these Memory saving methods (especial Compression and Swapping) hits your VMs you can be sure there is too much memory overcommitment. With the current server sizings and memory prices, in most ent. environment memory overcommitment is not usual (or only on a small scale).
You already have the exact problem point, your developers are not happy. For exact numbers check the Memory Performance for Ballooning, Compression, and Swapping of the VMs.