Problem:

Is there any tuning parameters for rebuild that might help performance?

Resolution:

During processing Rebuild sorts a temporary file which it generated when it loaded the indexes (the file is created in the system temporary directory) and then uses the results of the sort to update the indexed file. There is an environment variable - SORTSPACE which specifies the amount of memory to be allocated to internal workspace for SORT operations

The setting for SORTSPACE when performing a rebuild should be based on key size rather than record size (as it would be when using MFSORT). As a rule of thumb the value of SORTSPACE should be the larger of the following values rounded up to a suitable megabyte:

a) SORTSPACE = (number of records * (size of the largest key 16))  

Or

b) SORTSPACE = (number of records * ((number of keys with duplicates * 4) 16))