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Using runrepeat listings, your flyknit racers are 130g lighter than the speedcross. A very different trainer.
As @Sainsburys_Ed mentions they're good hikers too, I also wear mine for gardening too & also long stints on feet on a hard (painted concrete) surface at work on which road shoes quickly give me sore feet. I do run in them too still.
If you want a really light but grippy trail shoe it may be worth a look at the inov8 x-talon 212, as name suggests 212g. They'll wear out quicker on harder surfaces though if owt like my 190's (my xc shoe past 2 seasons, sadly discontinued).
Slower vs what trainer? Like4likeroute? Road or trail? Conditions? Heavy week? Sh1t day?
So many variables it'd be hard to say off one run.