There are a few factors to consider:
1. The access pattern: The SSD read cache is mainly designed and implemented to raise the random access speed of spinning disk arrays. Therefore if the access pattern of your application is mostly sequential, using SSD cache would not bring much benefits.
2. The SSD read cache needs warm-up time. If the VD has not been randomly accessed much after the SSD cache assignment, then it's expected that the data has not been populated much into the SSD read cache yet, so that the performance does not have a obvious increase.