What DB2 environment do you mean? LUW or z/OS. I'm mostly z/OS, so I will give that view.
In z/OS you generally specify the Tablespace when defining the Table. You can also find by Selecting from SYSIBM.SYSTABLE S there is a column TSNAME.

As to improving INSERT performance, check your Freespace and Freepage. Also how many Indexes are defined. Each Index needs to be updated for every INSERT. In DB2 V8 and V9 there are Muliple...

You would be better posting to DB2-L, I think the IMS list might get upset.
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