Btw, Locus really becomes very slow and sluggish when a few tracks are enabled, even when they are not visible. That cant be right. Test case: Import single tracks of Gran Canaria from
http://http://forum.asamm.cz/download/file.php?id=478 , enable them all, then scroll around Prague. Very slow... and nothing to do with acually displaying any pixels.
Solution: a track should calculate and store its bounding box (max left/top/right/bottom coordinate) when imported into your database. Then you can quickly check if it's visible at all with just a few comparisons between screen and bounding box. This way, invisible but enabled tracks will not slow down Locus much.
Other solution: Store all your geospatial data in R-Trees. That will even work for tenthousand enabled tracks :-).