I notice that when I initialize the dcc get. mIRC basically freeze for couple seconds then the message "not responding" will appeared before it goes back to normal again. This is only happen on the first dcc get. I'm running on Window 7 Ultimate 64bit on one xeon x6560 with 12GB of ram. I know I don't any issue with the previous version

This is most likely related to UPnP which can be enabled or disabled in the mIRC Options/Connect/Options/Ports dialog. You can find a discussion on this issue here.

UPnP is extremely useful because it ensures that your IP address is set correctly for file transfers. It also opens the required ports on your router to allow you to send or receive files. Unfortunately, some routers can be slow to respond to UPnP requests, which may cause mIRC to pause for a second or two.

You can disable it in mIRC, however it generally makes life far easier, especially if you often have issues with your IP address being incorrect or you do not know how to configure your router to open the required ports for DCC transfers or chats.

I disabled everything related to the UPnP on mirc. It seems to resolved the issue as well improved the speed as well.

ok it seems that there still some stalling after disable the upnp. it seems to stalled happened when there is burst of download speed say about 400kbps+