What is von Neumann bottleneck?
Both are correct, though I'd say the first one more so.

A von Neumann architecture means the program is stored in memory along with everything else, rather than being a separate unit attached to the processor. This means that anything the computer tries to do, no matter what it might be, is bottlenecked by the connection between the processor and memory.

So "getting data from memory is slower than processing it" is a valid description, as is "you need to fetch instructions and you need to fetch data and both of those are slow". But the most important part is that fetching from memory is extremely slow compared to anything else the processor might be doing.