Bastard doors are doors that slow Link down from his usual 1.5px/frame speed to 1px/frame if certain directions are held when exiting them. Obviously you'll want to hold different direction when exiting them to avoid this slowdown, though it can seem counterintuitive at times.
You will notice that all of these doors face east/west. One important rule to know is that movingor out of east/west facing doors will always slow your movement unless the door locks behind you. This page lists instances where certain diagonal movements will also slow Link's speed.
Wq found that the doors only becomes a bastard door if you open it up with a key.
1st Popo Room Door
First bastard door in Desert Palace is this one going into the first popo room. Although it seems like you'd want to hold diagonal here to get to that first popo, you shouldn't as it will give you some slowdown in the doorway. Hold cardinaluntil you've cleared the doorway, and then go to to get to the popo.
Dark Fast Knights Door
This is the room right right after dark maze which has the two fast knights. You can opt to dash in this room, which is a little easier, but walking is faster if done correctly. If you do walk, you should be avoiding the bastard door here, which gives slowdown on. You'll need to hold here until clearing the doorway, and then go to diagonal.
Dumb Room Door
This room doesn't really have a name but whatever. This door is really stupid. You can't holdor without getting a speed reduction here. You have to hold until you've cleared the doorway before going to up and then dashing through the room.
Here you can't hold either diagonal (or ) without a speed reduction. Here I would actually advise taking the speed reduction, since you can beat the spike if you don't.
Mire Tile Room
You'd think you can just holdthrough this entire room, but that'll give a speed reduction in the doorway. Need to hold cardinal until you're out of the doorway, and then you can go to diagonal.