The main reason for the added strength of the slower epoxys is NOT that fact it soaks in further but because the molecular chains formed as it cures are longer in the slower curing mixes. This is what increases the strength. Of course, if you tried to put 5 minute epoxy that is already starting to cure in a joint, it will be weaker because of how well it binds with the material being joined so (it soaks in better) is partially true but not the full story.