Why so slow?

My thought on this is that something might have been preventing your CPU from being fully utilized. For example, software tuning, excessive number of background processes, or a discrete GPU whose memory throughput (in GB/s) is significantly lower than that of your system's main memory (RAM).

Speaking of a GPU, you failed to mention which GPU that you are using on that system.

ADDENDUM: I also took a good look at Bill (Gehrke)'s infrequently updated PPBM8 site for CPU-only MPEG-2 DVD export performance, and discovered that your CPU scored a relatively lousy 835 seconds. That's compared to my average of 521 seconds for a stock (or default Turbo'd) i7-4790K CPU.

If you are trying to multitask, that computer is not up to the task.

That's only a quad core processor and minimal RAM.

Moved to the Hardware forum.