My server is Dell Poweredge 1950. I have installed the ESXi 5.5 Hypervisor.
VMs with 64bit OS installed and 8 CPU and 16GB memory provided have really high CPU performance and performance is very slow. The same issue appears with XenServer Hypervisor installed.
Only VMs with 32bit OS installed appear to have good performance. Could you please advice ?
There are many things that may affect your performance. I hope you're also aware that 5.5 is an old and not supported version anymore.
I don't know exactly how the 1950 is configured, but I guess it's fairly similar to the 710, so your settings should look similar to these:
Please see this article (from where the screenshot above has been borrowed): Follow Up on Power Options for Performance When Virtualizing - Working Hard In ITWorking Hard In IT
Changing the BIOS as described in this article will make a big impact for your performance.
Also, I guess Hyperthreading is enabled so 8 vcpus would run on one of your physical cpus (4 cores, 8 logical cpus), otherwise you would have a monster VM with 8 vcpus.
The E5410 CPU is quite old and it does not have Extended Page Tables (EPT) feature.
With EPT, the access to the virtual RAM is managed by the CPU.
Without EPT, the virtual RAM of the VM will have to be managed through software (e.g. the ESXi hypervisor or type 2 hypervisor such as VMware Workstation 12.x, version 14.x and higher already requires EPT). The access becomes slow because the when the VM will have to either read or write a virtual RAM address, it will have to hand control back to the hypervisor, hypervisor translate virtual RAM to physical RAM, access the physical RAM location, hypervisor then go back to the VM. Just that description alone you can see how the overheads are incurred.
So for memory-intensive operations/workloads in a 64-bit VM, that is likely the cause of slowness that you see.