AA Performance in Benchmarks

[Shammah Chancellor via Digitalmars-d]

So, I was tooling around with one of the benchmarks I saw listed 
on twitter:

https://github.com/kostya/benchmarks/tree/master/brainfuck

This D benchmark spends most of it's time on AA lookups.   
Discussions about the implementation aside, I am curious as to 
why the D AA is so slow even after calling AA.rehash.

I took a look at the Nim tables implementation, and it looks that 
they use power-of-two vectors and use the property of primitive 
roots modulo n to get the next value in the case of a collision:

```
proc nextTry(h, maxHash: THash): THash {.inline.} =
   result = ((5 * h) + 1) and maxHash
```

So, my questions:

1) What are the general purpose performance implications of 
something like this?
2) I saw a thread awhile ago where someone had posted an updated 
implementation of the AA with benchmarks and showed that it was 
substantially faster in all cases.  I can't find the thread again 
-- what happened with this work?

-Shammah