There are a lot of random D&Ds that can actually help slayer. I forgot to mention them, so here goes:
- Monthly troll minigame. You can do the repairing the walls option instead of killing trolls, and you get the same xp for completing it. At 90+ I think it's like 40k xp.
- SoF and quest lamps
- Penguins, already mentioned.
- Fealty, already mentioned.
- Jack of All Trades aura.
- Daily task challenges, as long as they are easy you can do them with a ring of recoil/group venge/poison to get the xp from the kill as well as the boost from the Quartermaster.
- Joining a SW clan that can will 90%+ of the time will result in about 5-6k xp/hr.