Sorry for not replying sooner. [Note to other readers: Serge sent me an email about substantial problems with the accuracy of the tapped timings when tapping in SV.] To be honest, I don't really have an answer.
Tapping is implemented using a generic toolkit-level shortcut to a menu function, so it is very much at the mercy of the toolkit, windowing system, general system load, etc. There is no real-time guarantee or system timestamping for tapping event delivery, or anything like that. I don't understand why your results were so particularly bad, but even so I fear there may not be much that can be done within the constraints of your system to improve them, or at least I wouldn't know what.
Craig Sapp also did some measurements (on Windows -- I'm not sure whether you mentioned which platform you were using) of tapping performance and found that "long-term accuracy" was good but local quantization not so good (see http://www.charm.rhul.ac.uk/content/svtraining/svtour-20060630.ppt
). It's clear that he was seeing much better results than you are, although I consider the results he reports as "in need of improvement" as well.
To do substantially better than SV does currently, it may be necessary to bypass the GUI toolkit (Qt) for tapping events, but I'm not quite sure how best to approach that at present, at least in a cross-platform manner.
A more sophisticated and reliable method would be to use MIDI events for tapping. Of course this would require a MIDI input device (e.g. cheap USB MIDI keyboard) but would have the advantage that MIDI library code is available on all platforms that is designed to handle rapid delivery with timestamping where appropriate.
Needless to say, patches are welcome!