To confirm you do not see Photoshop CS5.5 listed in your Program & Features control panel?  Did you recently use some type of registry cleaner or utilizing the Adobe Cleanup Utility prior or after installing CS6?

Stephen4758 it is a choice if you wish to remove CS5.5 prior to installing CS6.  I would recommend completing the following steps though to allow you to successfully remove Creative Suite 5.5.

1. Remove Creative Suite 6 and any other Adobe Applications
2. Run the Adobe Clean up Utility instructions can be found at CS Cleaner Tool for installation problems | CCM, CS6, CS5.5, CS5, CS4, CS3 - http://helpx.adobe.com/creative-suite/kb/cs5-cleaner-tool-installation -problems.html
3. Check to see if you have any folders or files remaining in Program Files(x86)/Adobe and Program Files.  You may delete these remaining files.
4. Reinstall CS6 and enter your qualifying CS5.5 serial number when prompted.

The cleaner tool will attempt to remove the remainder of your Creative Suite but there will likely still be files and folders remaining.  If you use any third-party plug-ins you will want to preserve these prior to deleting unless you have access to their installers.

Also a piece of advice regarding the use of registry optimizers.  I would recommend always making a complete backup of your registry utilizing Regedit prior to it's use.  I have lost track of the number of users who have ended up running into difficulties after using Registery Cleaners/Optimizers.  Creating a backup is a great preventative measure to insure you don't have those difficulties.  Also the backups utilize very little disk space.

Sometimes users will keep older versions of Adobe software for compatability reasons.  If they are working regularly with users who are using older software versions they may need to have an older version installed as well for full compatability.  I know this has been the case with InDesign in the past although this issue has been more greatly reduced in CS6.

In addition users of third-party plug-ins or utlities might also still need access to the old version until the third-party vendor releases and update for plug-in/utility.

Finally there is the learning curve involved with adjusting to the most recent version.  If you are in the middle of a creative project with a looming deadline you may want to continue to use some of the older software until your deadline has passed and you have the bandwidth to become familiar with new workflows and processes in the latest version.

Hope this helps?  This at least is many of the reasons users have given me throughout the years on why they still utilize or wish to retain the older version.  Personally I keep a couple versions back installed but that is mainly for support purposes.

No unless your hard drive is full.  If so it is not the storage of the install files but rather the general issue of not having sufficient disk space which would be causing the slow performance.