Slow query respons

Hi, I am using Informix OnLine5.0? on SCO ODT5.0 (single 133 MHz CPU, 2x1GB disks, 32MB). When I query table with approximately 250 000 rows, query runs very fast - few seconds (sqlexplain.out shows index path and estimated costs=5). But, when the same query runs more then one user simultaneously, query takes about 5 minutes (sqexplain.out shows again index path and estimated cost=5). Any ideas? +--------------------------------------------------------+ | CC Computer Consulting d.o.o. | +--------------------------+-----------------------------+ | Zeljan Silje | e-mail: zeljan@laus.hr | | CC Computer Consulting | Tel: +385 (20) 411-136 | | Janjevska 15 | +385 (1) 6552-330 | | 20 000 Dubrovnik | Fax: +385 (20) 411-136 | | Hrvatska (Croatia) | URL: http://www.laus.hr | +--------------------------+-----------------------------+