The difference is much worse. dd is not a good tool to measure performance if you want the results to be anything near what is happening in real life. dd performs a long sequential write, and that is not what your Discourse server is doing.
We use fio for benchmarking stuff like this.
See this article: https://www.binarylane.com.au/support/solutions/articles/1000055889-how-to-benchmark-disk-i-o
The base SSD disk is around 9 times faster than the add-on volume.
Base disk:
Jobs: 1 (f=1): [m] [-.-% done] [115.8M/39880K /s] [29.7K/9970 iops] [eta 00m:00s]
READ: io=307668KB, aggrb=127346KB/s, minb=127346KB/s, maxb=127346KB/s, mint=2416msec, maxt=2416msec
WRITE: io=101932KB, aggrb=42190KB/s, minb=42190KB/s, maxb=42190KB/s, mint=2416msec, maxt=2416msec
Add-on volume:
Jobs: 1 (f=1): [m] [100.0% done] [11484K/3904K /s] [2871 /976 iops] [eta 00m:00s]
READ: io=306676KB, aggrb=14174KB/s, minb=14174KB/s, maxb=14174KB/s, mint=21635msec, maxt=21635msec
WRITE: io=102924KB, aggrb=4757KB/s, minb=4757KB/s, maxb=4757KB/s, mint=21635msec, maxt=21635msec
On the other hand, don’t overestimate the impact of a not so fast disk: if you use the add on disk only for files (public/uploads ) then nginx caching and kernel filesystem caching will save your day and you’ll still have a pretty decent performance.