Rendering in this context means producing an element tree, or a textual description of one, which is capable of firing off many little (slow) allocations. The implication being that this will not scale to a large number of nodes without slogging the machine unduly.
If it's in memory and off-screen, i think it won't be that slow even for many elements. The actual issue might be the garbage collection pauses, which might become noticeable.