Can you login esxi with ssh ,run esxtop to troubleshooting.
may be those resons possible more likely :
1、storage performace issue (check the %VMWAIT)
2、assign too much vCPU (check the %CSTP )
3、CPU limit setting (check the %MLMTD)
and the other reson , like whether vmtool is to install or out of date ....
If you have assign all your VM's with Single CPU single Core then you should at-least change it for the VM's having high count of CPU's.
Like if you want to configure VM's with 8 CPU then would recommend to configure as 4 CPU with 2 core or 2 CPU with 4 core.
So in you environment 32 CPU can be assign as 4 CPU with 8 core.
If you are having 8 CPU to single VM with single core then it will wait for the vmkernel to release all the 8 CPU to complete the task but if you are increasing the core and not CPU then it will act fast because of hyper threading.
You can try changing the CPU and check the performance