Hi Jay,
In regards to the settings for LPR, the default value of the number of frames that our LPR software analyzes per second is 4fps. However, that value is adjustable and doesn't mean the camera has to be set to 4fps. The camera can be set to anything higher than that value. If the LPR is for a situation where the car has to drive vary slowly (e.g., entering a parking lot or parking garage) or even come to a complete stop (e.g., a parking lot/garage where they have to wait for the arm to raise), then you could easily get away with analyzing fewer frames per second because the car will be in the FOV longer.
You will have to do some "trial and error" because LPR definitely isn't a simple plug-n-play technology. It takes some tweaking and tuning. I would recommend reference the LPR second in the administrators manual for whatever XProtect software you are installing. There is a lot of information in there covering angle of view, pixels on target, shutter speed, etc.
For the SQL Express error you got, it is a fairly generic error from SQL that can mean a number of things from permissions issues, to a setting in SQL that is preventing our software from doing what it needs to do (this could be the case if SQL Express was already installed by a different application and you are trying to reuse that instance), etc. Installing one VMS on a machine with another VMS almost always causes problems. If you haven't solved this already, I would suggest opening a case with tech support so they can take a look in the logs to see if there is further information surrounding that error.
Also, as Josh mentioned, the OS drive being full can cause very weird behavior in Windows and can definitely cause things to run incredibly slow.