Hi All..
According to my humble understanding..
First you have to add more vCPU to give the VM more CPU power then reserve the same amount on its resource pool (as I think if you reserved a CPU more than what is configured as vCPU it'll not be useful). After that, you give it high share of CPU and an appropriate limit which doesn't kill the other VMs by getting all of the Host's CPU.
Finally, observe and see what the result is on the performance of this VM and other VMs on that Host, fine tune the reservations, the shares and the limits till you can reach the targeted performance...
Best regards, 