Ways to speed up a simpletest?
There are few ways of speeding up the Simpletests:


Consider upgrading your PHP to >5.5 with cache benefits of OPCache.
For PHP &lt;=5.5 try to implement PHP caches such as XCache, APC, eAccelerator.
Install and configure memcached.
Patch your Simpletest to exclude testing of disabled modules.

See: Tests of disabled modules shouldn't be parsed, Strengthen tests by not autoregistering non-test namespaces of disabled modules
Use SimpleTest Clone

(consider patching the module to exclude some big tables while cloning).
Optimize your MySQL instance e.g. via MySQLTuner-perl.
For sites that have lots of slow queries, disk access is often the bottleneck. So speed up by moving MySQL temporary directory to tmpfs., How to setup mysql on tmpfs?
Test only specific Simpletests groups that you need.


Other option to consider:


Try to identify the bottleneck and upgrade your hardware.
Break down simpletests into groups and run on separate machines.
Forget about the speed by integrating Simpletests with Continuous integration (CI) such as Jenkins, Travis CI, AWS, etc.
Use alternative test frameworks (e.g. PHPUnit), see: [meta] Replace the testing framework with PHPUnit and possibly rewrite Simpletest.


Use alternatives:


Behat (recommended)
PHPUnit (which has been added to Drupal 8)
Selenium web driver or Selenium IDE (which is slow as well:)