If you've ever used a Small Stone, you know that the last 1/4 turn of the rate pot goes into an unusable territory. Fast, warbled modulation that is not musical or even suitable for weird-sounding effects. I searched the internet looking for "rate mods", and couldn't find any simple answer of how to adjust/fix this.
It's actually really easy on the LM13600 version.
Right off the #3 leg of the 1M rate pot is a 27k resistor. This simply has to be increased to a 68k resistor. Once you do this, your max rate setting will sound more like a really fast Leslie speaker, rather than a really bad Chinese gong. I think this also gives you a slower sweep at the minimum rate setting. Very good. This should have been the stock setup IMHO.