According to this Quora post,
It refers to two things: A systems bottleneck, in that the bandwidth between Central Processing Units and Random-Access Memory is much lower than the speed at which a typical CPU can process data internally. The 'intellectual bottleneck' in that programmers at the time spent a lot of time thinking about code optimisation to stop 'lots of words' being pushed back and forth between CPU and RAM.
But according to my class lecture, it is the problem that "Generic implementation of single, common instruction and data hierarchy makes it impossible to access data and instructions at the same time. This is traditionally called von Neumann bottleneck."
Which one is correct?