If you look at the status bar you'll see "Cache X/200" This represents the number of blocks of data in the cache. If it hits "0/200" (it's like a gas gauge). It means Newsbin has used up all of it's buffers and it waiting for disk IO to finish. That's a sign your disks are slower than your download speed. Either because they're busy, because they're just slow or because something is getting in the way of writing.
You can adjust the size of the Chunk Cache by adding a variable in the .nbi in the [Settings] section:
The default is 200.
Memory Cache Limit
Files are downloaded and then processed by AutoPAR in RAM until the decoded file is written to disk, up to 20 MB by default.
Files larger than this setting will download to disk, the old way.
You can adjust the size of the Memory Cache Limit by adding a variable in the .nbi in the [Settings] section:
The default is 20. Setting MemCacheLimit=0 will turn it off.
It works at > 20 Mbps but, the slider isn't calibrated. That means what you set isn't exactly what you get. You might have to jerk with it some to get the speed you want.
You need to have Newsbin expanded to at least 1280 pixels in width to see all the items on the Status bar