It depends on several factors:

1. The number of rows returned in the answer set of the query. (does the
answer set grow as the databases size grows).

2. Whether the access path used is an index or a tablespace scan (does DB2
have to read all the rows in the table to answer the query?).

3. How you have DB2 configured. What is the size of the DB2 bufferpools, the
tablespace configuration, the DB Config, etc.

If you post the exact DDL of the table (including the indexes) and the
Query, then we could probably give you some idea of what to expect. Also
include the number of rows now and in the future.

It is certainly possible that some queries could perform just as fast with a
very large table as with a small table. But the above information is needed
to determine whether it is true in your case.