MyISAM is frequently described and marketed as providing fast
reads when it really provides fast index and table scans. This is
a more narrow use case as fast reads implies great performance
for most queries while fast scans implies great performance for
single-table queries that are index only or do a full table
MyISAM caches index blocks but not data blocks. There can be a lot of overhead from re-reading data blocks from the OS buffer cache assuming mmap is not used. InnoDB and PBXT are 20X faster than MyISAM for some of my tests. However, I suspect that mutex contention on the key cache is also a factor in the performance differences.
While there are many claims about the great performance of MyISAM. There are not as many examples that explain when it is fast. Alas, the same marketing technique is being repeated with NoSQL to the disadvantage of MySQL.