I once rode 30 km with 65 kg loaded on rear basket and along (square tubes) the horizontal tube.
It's just pushing harder and riding slower.
It was on the outside. In a city, alot stops and starts, it requires a degree of crazyness. See, crash = lotsa damage, bike and rider. The heaviest load I ever went with through a city, a couple gitar speaker boxes and their amplifier, when arriving home the only sane thought was a never again. Realizing having had a lot luck by all green traffic lights and no surprises. Happy to succeed, but at the same time, realizing what risk it had been.