Without knowing the dimensions (resolution and size) of the existing video, what program you are using to convert it and what settings you have set for the converter/conversion, I'm not sure how we are able to compare it to anything. I'm also confused your choice of word "decoding". Ripping a disk is not processor intensive, so I think you mean encoding/converting/compressing.
To give a very rough comparison, I can convert uncompressed DVDs to MKV files ("20" quality compression in Handbrake) in around 15 minutes each. I can convert uncompressed Bluray to MKV ("22" quality) in just under an hour per movie. This is on a Core2Quad QX9650 running at 4.0 GHz.