slowing a movie down takes a lot more than frame blending - if you have not used something like twixtor - i suggest you give it a go - to see what is really involved in "slowing things down" - its a little more tricky than dissolving...
http://www.revisionfx.com/products/twixtor/overview/
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sure- its not particularily difficult to do frame blending but it has to fit in with the other things i do and not kill performance
i have concentrated on many different things in the last couple of years - 32 layers and 32 mixes and 12 video inputs in current preview software and current computers can do 8 layers of hi-def 1080p without problems- with real world colour correction type things
features have to be performance neutral - because a lot of users push things very hard and the computer is already at its limits