Tell us more about your setup.
There are many reasons why a VM appears slow/laggy and usually it's not the network. The first things I'd check are CPU overprovisioning and I/O latency first. You can do that via ESXTOP on the ESXi console.
High CPU Ready % and Co-Stop indicates CPU overprovisioning (VM is ready to run but cpu cores are not available)
KAVG/DAVG/GAVG indicate storage latency (KAVG = VMware Kernel, DAVG= storage, GAVG=KAVG+DAVG)
Depending on yout setup the storage just may not provide enough IOPS.
See here for some info about ESXTOP: ESXTOP - Yellow Bricks