Any Virgin Mobile branded firmware is going to be slow, rooted or not.
As long as your mobile provider in Croatia supports 850mhz/1900mhz then the i9000m will work fine. Head over to the development section, download a ICS rom (I prefer CNA personally), flash a modem more suited to your cell providers, and you're all set.
If you need more info, tell us who your cell provider is.
HTC One M7