Why is password recovery so slow for Whirlpool hash, when compared with others?
how much faster can i recover my password (20+ characters) from
  knowing nothing about it (it's been years since i last used it) using
  a gpu instead?


According to this benchmark, a single Nvidia Tesla V100 (the best GPU commercially available) will hit 94kH/s on that specific mode (search for 6231 on that page) which is about 300x as fast as your CPU and of course you can just use multiple GPUs. For example on Amazon's cloud services, you can have up to 8 such GPUs per VM.

Note however, if you really used a random 20+ character password, there are at least $2^{94}$ possible passwords (if you have used 20 lower case characters only) and testing this amount of passwords is infeasible (and would require $2^{77}$ GPU-years).


  I'm just curious about why bruteforce takes so much longer on the
  whirlpool hash than it does on the others


This is hard to answer. It may be that not a significant amount of effort has been invested in optimizing the Whirlpool implementations, because it's a somewhat rarely seen hash function. It may be that the huge amounts of table-lookups that Whirlpool apparently requires slow down the operations imensely compared to the register-fitting other hash functions.