When you look in task manager is the load spread evenly between two cores or are 3 cores running at full and the rest idling? What hard drive do you have? That is often one of the slowest parts, especially for large items like bluray movies, it will likely have to rip to HDD, then move to RAM, convert, and send back to the HDD. Check what your memor usage is like while its running, if its only a 32 bit program it wont be able to make use of all 8 GB of memory but if its using the full 2GB allowed for a 32 bit application its going to be calling back to the hard drive alot.
Sounds like a bandwidth issue, either memory or io (disk). There's not much you can do to relieve this with your system, it's just a matter of your cpu being able to process it faster than data can be moved back and forth. If it's being read from a blueray disk the disk drive is probably the bottleneck.
Most blu-ray drives are slow. The bottleneck is almost certainly the read speed of the drive itself.
No. Perhaps i was not clear enough. But in the message preceeding your message, i told that when i try the conversion from my harddrive (SSD), i get the same results as when i try the conversion directly from the bd.