Lvm Cache Does Not Give Any Performance Enhancement

I'm using Ubuntu 16.04.4 and it's running on a VM. The disk that has the origin data has an I/O limit on 1MB/s and disk for cache has no limit. I have followed the guide at fibervillage to create the LVM and succeeded without errors, with the modification to cachemode writeback, and I also did try writethrough just to try.

When the LV is without cache I got the expected behavior with 1MB transfer rate, But when I added the cahepool to the origin LV I do not see any performance enhancement at all. But creating a LV on the same disk were the cache pool is I get way faster transfer rate. I mounted all volumes in /etc/fstab with sync,dirsync options to avoid any other caching.