Less than I expected them to, but I definitely take tight turns more slowly when they're on.
It's more a weight distribution thing than anything; if I've got 30 lbs of gear in those rear bags and nothing up front, handling gets super hairy on hills, gravel, or rain/snow. Fishtailing on a bicycle at 20+ mph is fucking terrifying.
If the weight is spread out, it's significantly smoother. And the low center of gravity is great for gravel.

