You have to be more specific as slowness can appear at cpu,memory,storage,network so which layer you are seeing this behaviour of slowness.. Secondary is that is there a frequency on the issue ?
Basic checks and validate the below things
Check memory/CPU contention
If there is any recent changes in the environment for that VM, validate the same
Power management on ESXi host VMware Knowledge Base
Are there any other jobs running on the VM when there is a slowness observed?
Snapshots?
Thanks,
MS