I committed a small test runner tonight that does 10000 adds, multiplies, subtracts, and divides with BigNumber, Long, and Int (as a baseline). These are the results I'm seeing on my dev machine:
(you can run it with this commandline: node longperftest.js)
488.002ms BigNumber
5.058ms Long
0.330ms Int

I think this effectively rules out BigNumber . [Looking at its implementation, it's slow because it supports arbitrarily large numbers by storing them as strings and then converting them to digits to perform operations on them. In addition, it doesn't support the bitwise operations that we need]

I also looked at the goog.math.Long implementation and it seems quite optimized. It is storing the high and low 32-bits in two 32-bit integers and then uses integer math directly. I don't see a lot of potential for optimizations. Unfortunately, JavaScript only natively supports 64-bit floating point numbers, so it's 64-bit integer math will never be as fast as languages that support 64-bit longs natively.

Regarding the slowness of recip, I'm doubtful that we will be able to get it much faster. If the intent is for having this code run in the browser, I think we're stuck. JavaScript servers (like node.js) would probably implement this logic in C/C++.