Galaxy Gio doesn't find GPS satellites after minutes of searching

I recently bought a Samsung Galaxy Gio (GT-S5660). When I try to connect to the GPS satellites, 75% of the time the built-in GPS is unable to locate me within a few minutes. I keep seeing the blinking GPS icon. The other 25% of the time I get a GPS signal within 15 seconds. Is this unusual behavior? If yes, what can I do to find the cause of the problem? Are there any known "death grip" issues related to GPS chips? I should note that I don't have a data subscription for my phone and I've disabled mobile internet on my device, but I have offline maps (certain areas in Google Maps and my entire country in MapDroid) so the GPS chip should be able to operate without data or WiFi.