Your edits do improve the question, but you are still asking two very different questions.
We don't really need to answer these questions:
How fast are ions transported passively inside the dendritic tree (and soma), probably depending on their atomic mass, charge, and of the local geometry?
What is the order of magnitude of the speed of passively transported ions in the dendritic tree?
to answer this part:
Especially I want to know when a post-synaptic potential generated x μm away from the axon hillock "arrives" there, i.e. can be detected/summed up with other PSPs
The speed of propagation of electrical signals in neurons don't depend on particular masses of particular ions or diffusion per se. Instead, passive electrical conduction is best understood using the cable equations which treats conductance within neurons as electrical RC circuit. The electric field propagates at the speed of light.
Therefore, EPSPs actually propagate almost instantaneously (given how short the distances involved are relative to speed of light) in that they produce an electric field. However, the peak decays with distance, and the rise time of an EPSP depends on the compartment because of capacitive filtering along the length of the dendrite. The decay peak is described by the length constant:
$$\lambda = \sqrt{\frac{r_m}{r_i+r_o}}$$
In this equation, rm is the resistance of the membrane, ri is the resistance of the intracellular space, and ro is the resistance of the extracellular space and is effectively negligible.
The length constant is the distance at which the peak amplitude will have decayed 63% from the peak at the signal origin.
The temporal filtering comes from the capacitive properties of the membrane along the length of the dendrite. The authoritative works describing these properties are several papers in the 1960s from Rall, Rall, W. (1962). Theory of physiological properties of dendrites. Annals of the New York Academy of Sciences, 96(1), 1071-1092. and Rall, W. (1969). Time constants and electrotonic length of membrane cylinders and neurons. Biophysical Journal, 9(12), 1483-1508. are good references.
The effective time constant for signals originating closer to the soma is much faster because there is less membrane to charge along the way. Exactly "how far" you have to travel to make a certain amount of difference depends on the properties of the membrane, which vary across neurons, vary in different parts of the dendritic tree, and vary dynamically within a neuron due to synaptic transmission.
Magee, J. C. (2000). Dendritic integration of excitatory synaptic input. Nature reviews. Neuroscience, 1(3), 181. is a fairly accessible review that covers a lot of these issues and more. It's important to recognize that although passive propagation of signals is important in dendrites, it's never purely passive and active conductances have important roles as well; in many cells, these active conductances plus differences in the initial shape of EPSPs throughout the dendritic tree act to mitigate the effects of distance.
See also Koch, C., Rapp, M., & Segev, I. (1996). A brief history of time (constants). Cerebral cortex, 6(2), 93-101. for more on the time constant and some evolution of the ideas about neuronal time constants since Rall.
