I am not exactly sure, but could you just move the call to ArchGDAL.getgeom(feature, 0) out of your for loop. Because at the moment you are newly loading the geometry of the timezone for every point. And these geometries are only freed after your do block for the feature.
I would also convert the points into ArchGDAL points before your first for loop because otherwise you have to do this for every timezone anew.