The very bad rates of climb were on the first versions. As anyone will tell you a twin climbs better than a quad.(engine out performance,. , etc..)
Problem was that controllers were getting used to twins on long range and expected the same performance or better for the 340, it was not and by far .
But still far better than a DC8 or a 707 for those old enough to remember.
Taking off and staying airborne due curvature of the earth was a common joke back then.
Anyway the 340 is disapearing ( not produced anymore) the 350 will climb better than a 330 , so old era.