MYSQL parallel query performance issue
Upgrade to 5.5, then 5.6, then 5.7.  You will be able to run more simultaneous queries at each upgrade.  Do you really mean "5.1.1"?  That was a pre-release for 5.1 from about 2005.
Use summary tables for old, static, data.  (It is static after it is INSERTed, correct?)  Perhaps summarizing (eg AVG, MAX, MIN) daily readings can be done just after midnight?
Use InnoDB, not MyISAM.  Much better sharing of the table.  (And it gets better with each version upgrade.)
innodb_buffer_pool_size should be about 5500M for your 8G machine.  Is the dataset, including indexes, bigger than that?  If so, get more RAM and/or RAID with striping and/or SSDs.
And Summary tables are the main cure for Data Warehouse performance issues.  Further discussion .  It's not uncommon to speed up queries 10-fold, especially I/O-bound ones.


"How does one solve..." -- Most applications are not doing lots of 3.5-second queries.  They are doing 'point queries', which might be 3.5 milliseconds.