Growing 1 single leaf in 2 months is normal as this is apparently a more slow growing plant.
Brown mark on bottom old leaves should be caused by previous freeze damage in winter. Won't be a problem right now as new leaves on top are shiny and healthy ~
However, it is a bit strange for the upper most new leaf ... those brown spots seems to be secreting glands but they usually won't show such colour and size ... They do look like bug's bite in certain degree although the bite marks are not looking familiar to me ...