At the speeds commercial aircraft are moving, yes, I could see a pilot mistaking a bird soaring for a RC airplane.
It has less to do with relative speed than relative speed vectors. The most difficult target to see, even when advised of its presence, is coming from a constant bearing decreasing range as there is little change in position with respect to field of vision. I have flown past, on three separate occasions, mylar balloons while at cruise flight levels (FL300-FL390) at 400-450 KTAS. The size and shape are arguably similar to drones, so distinguishing a drone from a bird while at drone operating altitudes should be equally trivial, assuming the event wasn't just a flash in the visual periphery.
Commercial aircraft are moving much faster, and the pilots are busy doing pesky things like preparing to land. Seeing a bird or drone is nothing more than a glance and a "I saw something".
My VMC scan at cruise is much less rigorous than while in a terminal environment. The flying pilot should be scanning outside (VMC), and leaving box work to the non-flying pilot. "See and avoid..."
All that said, I have not ever encountered a "drone" in my 15000 hours of flying, respecting the fact that I don't find myself in the environment (which most likely is not large commercial airports) in which drones operate. OTOH I usually have about 1 birdstrike per year, most of which I never see- only hear the impact.