high quality setting

Good question.
Why not do a one minute test from a single file and compare the two?
Find any local file and duplicate it (Command-D).
Open one and then the Movie Properties / Visual Settings tab, remove the check mark for High Quality and Save As.
Next, open both files, arrange the windows so both can be viewed and click "Play All Movies" (View menu).
What do you observe?
Note that the QuickTime Player preferences allow the end user to "Use high-quality video setting when available". Your "non" high-quality file wouldn't be available for this preference.

By default the high quality setting is enabled in any file you create using QuickTime Pro. You would need to disable the setting and save the file to change it.
The end user also has a bit of control via the QuickTime Player preferences by "enabling" high quality in files that contain those settings.
So. The settings are high quality by default for both the author and the user in a standard QuickTime install.
QuickTime Player (by design) will drop frames during playback so slower, older machines get "the idea" or the "intentions" of your video but still allow playback of the file. You could, in theory, decrease the likelihood of dropped frames during playback by turning off those high quality settings.
If you knew your audience was using older equipment QuickTime Pro allows you to reduce quality for each frame but enable more frames be displayed.
Does this make any sense?

Just the opposite.
High quality settings would increase the likelihood of dropped frames on older computers.
By older computers I mean really older computers.
Again, you can test these facts yourself using QuickTime Pro.
Open one your previous test files and then the Movie Inspector window. Position the windows so you can view both windows and choose "Play All Frames" (View menu). No audio when your choose this playback.
The Movie Inspector window will show the FPS: (source) and the Playing FPS (current settings).
Doubtful your newer machine would show any difference but it could be dramatic on an older one.

As to what QTK said... I have done a few tests with the preset settings in Quicktime. I found that they are not that bad... If you don't know how to set your own settings. These settings are a good starting point.