Relatives is not just fast, it also features a deliberate innovation; its perfect handling of non-standard tags. Yes, perfect.
The GEDCOM specification allows programs to ignore unrecognised tags and many do so. Some other programs make the serious mistake of molesting your database by inserting comments for each unrecognised tag. In the first case, the unrecognised tags are lost after importing and exporting again. In the second case, the tags are lost and your database is messed up with weird comments. You do not want either.
Relatives does the right thing; it does not ignore the tag by
throwing it away and it does not molest your database by inserting
comments either. It will simply import each unrecognised tag as a tag
and - this is the crucial bit - will export it again later. This
enables round tripping of vendor-specific GEDCOM extensions. What's
more, on export, all invalid GEDCOM extensions are changed into valid
ones by prepending the mandatory underscore, to avoid creating invalid
GEDCOM files.
If only all genealogy programs were as smart about handling non-standard tags...
Tamura Jones