How much more? It surely doesn’t take that much more speed to keep even 60 kg in the air —assuming the feathers are congruently scaled; if not, then Gwaihir and his kin will more resemble azhdarchids instead of eagles.
But in any case:
Mordor has fell beasts. They fly.
Consider, too, that the Nazgul often came close to the Ring, but never seemed to spend a lot of time looking for it in a lot of other places. That suggests that the Ring, while not a homing beacon, could still be effectively hidden, just not absolutely so. So Sauron tended to concentrate on strongholds of power as likely places to hide away the Ring (hence the siege of Minas Tirith).
Gwaihir may also have balked at murdering hobbits by dropping them into volcanos (probably not something they’re used to eating, anyway), but I think it was mostly that flying to Mordor would not have worked, not with a huge surveillance camera atop Barad-dur. Consider that there might be radio/telepathic linking between Sauron himself and his air force, that’s likely a fairly effective guard against any invasion of Mordor’s airspace.
When you also consider you’re asking giant eagles—but mortal giant eagles—to basically fly into an evil god’s territory, well, that speaks for itself.
