What makes Haskell programs slow down?

Write section in Forward.

Relevant part of the Proposal, I've tweaked some stuff and we'll probably need to refine this section
```
3. What makes Haskell programs slow down (in order of importance)
3.1 Doing more work than necessary <=> too much allocation
3.2 Fusion and why it sometimes fails
3.3 Too much polymorphism
3.4 Bad datastructures
```