SLR either is intentionally trying to limit it to a single download per IP, or has seriously broken bandwidth management code. There is nothing wrong with the users set up and page file has nothing to do with this at all.
This is a problem with SLRs content download system. I see it any time that I try to download more than one file at a time on one IP. There are also reddit threads and other posts with users confirming this.
Re download managers:
I dont have time to dig into the code, but it appears this is intentional from SLR. Its not a bug in the DL managers. Ive tried many and the last one was Chromo Download Manager.
If you set concurrent > 1, it works but then SLRs "broken" code to limit to 1 kicks in and in most cases ends up borking all concurrent downloads. I wish they would just limit it to 1 and not even try to start the secondary ones.
When concurrent = 1, the problem seems to be is that SLR is intentionally generating short lived temporary download links. So if the DL manager starts them right away, not problem. But if it waits, like when concurrent = 1 and and existing download is in progress, then when it gets to the queued ones the links have expired. Thats why it returns and HTML page instead of an MP4, and is probably why sometimes even on the site clicking links on pages that have been open for a while generate invalid mime type errors as it returns html instead of mp4, but the mime type was already in the header.
Im not trying to scrape the site or anything. Its just that I have 2 internet connections, both not very good and I live in a remote place. So streaming always causes lower quality and often is like watching youtube in the early 2000s. Lots of pauses for buffering etc... thus I download... and this is quite frustrating to say the least.
I dont see how any download manager can work with this set up short of using something like Power Automate or some other scripting tool to mimic clicking of the download buttons. Far more work than Im willing to do and I guess I'll just have to click download links one at a time in the background while Im working on other things.