This occurs in all of your clients.
If yes you could check where your tables were analyzed in the dba_tables. Check too if your tables are fragmented ( if this occur you could reorganize this tables), and check if your queries are using the correct indexes.
This is nothing to do with the Database performance. The connection problem could be because of some network issues. try to ping the server from your client and see the delay. Secondly, i hope the configurations of your client pcs are quite good.