Indexing will look at every record's value while building the index. So I'd imagine the number of records is causing it. IE, you're not indexing 7000 record values, you're indexing 150,000 record values which equates to a list of 7000 values.
How are you storing the field's index? Is it set to store "all"? (vs. minimal or as-needed). It might be worth it to turn on all storage for this one field. It will increase your file size, but cut back on the initial index creation it seems to do the first time that field is used.
Here's a filemaker article about indexes for reference: