The chain running slack like that during backpedalling indicates some stickyness in the system. It can be from the sticky packing grease in a new chain or grunge from a hyper dirty chain or it can come from grunge drag in the derailleur pulleys or drag in the freehub. Almost anything that causes drag can cause this slack to show up. I think it's what the mountain bikers call "chainsuck" since it happens when things get muddy or the parts are highly worn out.
In your case I'd suggest you start with cleaning the chain to clear out the packing grease and use a nice oil instead. If the issue is still there then check for the freehub (if you're using a cassette cogset then you've got a freehub and not a freeWHEEL) With the cassette in place if you give'er a snappy backspin it should spin a good 6 to 10 revs before the freehub ratchets and drag pull it to a stop. If it only goes a turn or two then the new freehub has some of that heavier grease inside it. Either live with it and it'll free up over time or dribble some light oil like Triflow or air tool oil into the gap between the splined shell and the dust cap so it goes into the freehub body.
To do this right and without getting the axle bearings washed out with the oil you'll need to remove the axle and dust cap so you can get the light oil down the gap between the splined shell and the axle bearing cup. If you've got a sealed cartridge bearing hub instead of a standard cup and cone then you'll need to look for a similar gap depending on the type of hub. Get a good 1/4 oz or so down there and spin the cassette frequently to encourage the oil to mix with the grease. Depending on what freehub level you have the excess will either run out the base and over your hub or it'll stay in. Some have a seal and others don't. If it stays in then after it seems to be running with less grease drag tip it over and leave it sitting cassette down over a dish or bucket to drain out the excess. Leave it for a couple of hours like that. Then put it back together with a good water resistant grease in the wheel bearings and you should have a nice lively system.
If you're getting this parnoid about the these things it's likely time to take apart the derailleur pulleys and clean and lube those as well. If the one has a ceramic bearing then just clean it and don't lube it. Or at most use some of the very light body oil on it. The other metal bushing idler should get a little grease.
Do all this and you won't have that chain slack anymore. And you'll have gotten back that last 1/10 of a joule of energy that was being sucked up by the viscous drag of the greases..... hey! Don't knock it. Over the course of a week or two that adds up to a whole donut! ! ! ! !
