ADOQuery very Slow on ISND 64Kbits line (DB2/AS400)

Why does it take a bit more than 3 seconds to get a result set of 2 KByte through a ISDN line at 64 Kbits ?
The server is DB2/AS400

Here is the Query : (Cursor location : Server; MaxRecord = 20 )

SELECT Col1, Col2, Col3, Col4 FROM MyTable

Col1 = VarChar(10)
Col2,Col3, Col4 = VarChar(30)
Total for each line 2000 bytes (maximum)

In fact  The first time the query is triggered it takes more than 3 seconds. The second and third time 1 second ++.
In my mind, it should take less than 1/3 of a second . It seems that each layer (ADO, OLEDB,...) spends time to do something, but what ? Probably buffering ? Anything else ?

The same query has been triggered through "IBM AS/400 Operations Navigator" (which does not Use ADO/OLEDB). It takes less than a second.

Is there anybody who can explain ? Or give me a solution.

Thank's in advance.