Hi Fizzer,
Thank you very much for the valuable information and sorry about the late reply.
I needed to schedule a maintenance window in order to be able to turn off all the servers in one of the hosts and try that configuration.
I did that yesterday and it was a heck of a day.
We're still running version 5.5 of ESXi on the 2 servers we have and version 6.0 on vCenter in the management server.
After doing the configuration on VMware server 2, I had to reboot that host and that was the beginning of my nightmare.
I wasn't aware of a bug in that version of ESXi when using iSCSI that makes the restart realy slow.
When I restarted the server it wouldn't come back, then after 45 minutes of waiting I decided to go to the company office.
When I arrived there, the ESXi loading progress bar was at around 5% and I had to wait for 5 hours until the system load completely and go back online.
Then I turned on all the VMs and everything went to normal again.
Today morning I did some testing comparing the VM in the host where I disabled the DelayedAck (TestVM02) and the other one with that enabled (TestVM01).
To do that, I downloaded a 20GB file from a NAS to both VMs at the same time (picture below).

Then, I opened vSphere -> Performance for each VM and compared the 2 servers to see if TestVM02 had an improved performance (picture below).

So far, the results seem inconclusive for me.
Both graphics seem to have similar patterns and vary on the same range (observe that the axis scale in each graphic is slighly different).
However, there are some numbers (marked with a red rectangle) that are very different on each host.
Now I need to understand what exactly those numbers represent, and if those mean that the performance would be better on TestVM02.
We're planning to move one of our real VMs from VMware server 2 to the storage and check how it behaves, but I still want to do more testing before.
Please, let me know if you have any other idea that could help to improve that performance.
Thanks again!!