Hi, VMs may slow due to some resource related issue.
1. CPU- in VM you need to crosscheck if it is consuming 100% cpu. If the CPU utilization is normal in VM thn you need to crosscheck in vm performance graph if there is high ready value for that particular VM.
2. RAM utilization and also need to make sure that swaping is not happening. you can also croscheck this from performance graph.
3. DISK IOPS- login in VM and open performance graph (open task manager go to performance tab thn click on open resource monitoring option.), in disk tab you will be able to see the disk queue length and it should be less thn 5 to get the required performance.
let me know the details so that i can suggest you further on this.