To connect to the schema tables, the connection can last 5-8 minutes.
Do you mean that this is the elapsed time between the moment you open a table and the moment you see the result on your screen as a map ? And does that include a view of all the 3600 polygons in that table ? Or does your view contain multiple tables ?
Obviously fetching many geometries from a database will take time, but it should not be that long. For example, fetching some 3200 fairly complex polygons is less than a second (say 800ms) on my database. This is not using QGIS - just a simple fetch. But the cost for reading from the database, passing to the client over the network and decoding the objects is the same irrespective of the client.
So there is definitely something wrong in your case.
Can you try accessing the tables using sqlplus ? Just do the following:
SQL> set autotrace traceonly statistics
SQL> set timing on
SQL> select * from <my_table>;
The first command tells sqlplus to not print out the results (that would be very long). Your DBA will need to grant you some additional privileges in order to to this.
The second command tells sqlplus to show you how long it took to execute the statement.
The third one reads the full content from that table. For example:
SQL> set autotrace traceonly statistics
SQL> set timing on
SQL> select * from us_counties;
3230 rows selected.
Elapsed: 00:00:00.75
Statistics
----------------------------------------------------------
13 recursive calls
0 db block gets
1298 consistent gets
560 physical reads
0 redo size
4219623 bytes sent via SQL*Net to client
3930 bytes received via SQL*Net from client
224 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
3230 rows processed
If this is also very slow, then the issue is probably around your connection to the server and you need to ask your DBA to solve this.
One thing that makes reading geometries expensive is their complexity, i.e. the number of vertices: the flattening of those vertices into messages, the transfer, the decoding of the shapes will all take time, and so will the time it takes QGIS to render them. But I would not expect timings of the order of magnitude you mention.
