Why does vifm display some kilobytes next to directories instead of showing the number of files they contain?
Displaying size of directory (i.e., size of file list) costs nothing and won't harm performance in any way.
How to display the number of files?
0.8.1 (beta for now) adds new option 'dirsize', which can be set to nitems to compute number of items in directories (direct children, not whole subtree).
How to display the size of the directory (incl. all its children content)?
Use ga and gA keys. The first one will skip directories of presumably known size, the second one always traverses whole subtree.