See if http://fox.wikis.com/wc.dll?Wiki~VFP9RushmoreAndCodepage answers your question.
There's also a bug in VFP9 that causes indexes with STR() function in the index key to be ignored by Rushmore.

>At first I thought I was imagining it. Then I thought I must have changed my indexes.
>
>Finally I figured, I've still got FP2.6 DOS, lets compare that with vfp.
>
>That's what exposed the problem.
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>I wrote a little test prog in both environments which simply opens a large file on the network and does the same query in both environments on the same data and the same network.
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>The DOS environment returns a query with 21 records (from 170,000) in 1.3 seconds. Which is what used to happen in VFP8.
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>The VFP9 now takes 17 seconds to produce the same result. I've only noticed this slowdown since I upgraded to VFP9. And sure enough, some of my users are still running on the VFP8 version which still operates at the expected speed.
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>Just to add to the confusion, that query is based on a character field (which is indexed). If I perform a date based query, I get the same performance in both environments (and its much much faster eg 7000 records returned in under 2 seconds)
>
>Anyone shed some light on this problem?