Performance of mysql equi-join observed in HDD and SSD
I am excited to hear reliable comparisons between HDD and SSD, but first...

Here are some of the variables you have not accounted for.  (Or maybe you did, but failed to state.)


InnoDB?
One says "Using index", meaning that the processing happened only in the index's BTree ("Covering" index); the other apparently had to bounce between the secondary index and the data.  This could make a significant difference.
Table sizes?
Was the cache (buffer_pool) cold?  Please restart mysqld before each test.
Same CPU, since you want to focus on the I/O, not the CPU time.
If it was the same server, which drive was c on?
SHOW CREATE TABLE -- there could be subtle issues.
MySQL version.
Are the indexes aligned with the data?  For example:  AUTO_INCREMENT and TIMESTAMP might be virtually in lock-step.  But UUID is very random.


8x and 5.5x seem to be reasonable.