I've always used lxtask under both lxde and xfce as it works better for me.
However, I've just noticed that when running a large Virtualbox VM (Windows 10) lxtask shows the RSS and VM size as 16777216.0 TB !! Occasionally the RSS size will 'drop' to 2.0 GB. The vbox settings allow this particular VM to use (at most) 1912MB of base (real) memory; when base is set too high, bad things can happen to host responsiveness.
For comparison, Firefox runs with 351 MB RSS and a 1.6 GB VM size.
FYI my system has 4 GB of real memory ('RAM').
Anyone else see this symptom?