What speed were the old disks that the LUN was on? What speed are the new disks? When you expanded the LUN on to the new volumes did you migrate the LUN to the new disks, stripe the LUN onto a new LUN on the new disks, or concatenate the LUN onto a new LUN on the new disks?
If you moved from faster disks to slower disks, a slow down would be expected. If you striped the LUN with a new LUN and the new disks are slower than a slow down would be expected. If you concatenated the old LUN to a new LUN on the new disks (which would have been very fast so this probably isn’t what you did) then the old VMs would be the same speed, but new ones which are created on the new space would be slower.