As questioned above.
Not sure, but there might be a slight overhead if you want to pick those 1000 objects using families. Families has a very slight cost to performance, but not very much.
But it's more easy to work with instances than 1000 seperate objects.
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Other than that, the main difference is memory usage: Construct loads images for entire objects at a time, so one object with 1000 instances (and presumably lots of animations to cover all possibilities) would require all images always be loaded, whereas 1000 objects with 1 instance each allows Construct to load images for each separately depending on which are used. Basically don't create a single "mega-sprite" that covers lots of different kinds of objects; use separate objects and use families to avoid repeating events. Other than that, if there isn't an issue with memory usage, I'd expect the performance to be more or less identical.