Where is the speedup effect on using lvm2-cache

Question

I recently discovered that LVM2 supports now caching: http://man7.org/linux/man-pages/man7/lvmcache.7.html

The cache logical volume type uses a small and fast LV to improve the performance of a large and slow LV.

I tried it on two machines having a SSD and Harddisk, with the Cache on the SSD and the Data on HDD. But neither while reading or writing i could see any difference to the previous created LVM without caching. For example compilation of large C projects took the same time, even after several runs. What would be a use case for a LVM Cache?