Today, I decided once and for all to try and track down why it does this and I'm still none the wiser. Even if I disable my AV, it's still the same. I don't have any MS AV or firewall enabled anyway, but did double check the settings in case I made a mistake when setting this machine up.
I then decided to test using another reader - sabnzbd, and I have to say it works flawlessly. No slowdowns whatsoever. The problem is, I don't like it's GUI, nor how the files are saved. I'd much prefer to stick with NewsLeecher if I can... but these problems need to be addressed because I can't keep downloading files and having them pause every 5 seconds. I'm paying for decent broadband speeds, and a decent news server that can supply those speeds. I'm being let down by the news reader. See below for images.
First... NesLeecher. Completely inconsistent.

Now look at SabNZBd:

Devs - When or is this going to be fixed?
As much as I love using NL - I only have so much patience left in me.