Looking at the code, the entire file is read regardless of “Data file particle count” - so it’s not time to read the file, it’s the time required to set up the particle view in VTK. So this doesn’t point to slow file I/O but slow graphics. Are you running this under vglrun? Over a slow network connection, perhaps?
I’m working on speeding up the code - the easiest part is to remove the excessive calls to update_particles which are causing this to happen every time you change something in the GUI - the harder part is understanding why it takes ten minutes to render 285880 particles, since I cannot reproduce this locally.