Internet slows slows down when downloading large files for no apparent reason

Question

I download a lot of .iso files, mostly for Android-x86. However, they (and many other ISO files) randomly show down to 45KB/sec (300-500kbit/sec) when downloading.

I can't figure out what causes the issue. Windows 10, Windows 7, or Linux: still happens. Mirrored file on a different server: still happens. Different browser: still happens. Wi-Fi or Ethernet: still happens. My disk usage is at 0-8% when it happens. I have 3Mb/sec internet from CenturyLink (fastest speed where I live), and a ZyXel PK5001Z router (capable of 100MB/sec). Normally, files download at 325-350KB/sec.

If I let it continue at this horrid speed, it slows down to about 27KB/sec, then to 4.8/9.6KB/sec, and then will sometimes slow to a halt.

Speedtest.net does not reflect these speeds It shows them at near-maximum.

My Internet also jumps to 3.1Mb/sec when I run a speed test.

I do not use a VPN, and I am the only one on my network when it happens.

EDIT: The next day, I had no problem with the same download and mutiple devices on the network.

Solution

Most likely your ISP is throttling large downloads. Many of them do this. They give you a high speed for the first 10 MB or so to make speed tests look good and general browsing to seem quick, but after you hit the 10 MB limit (or whatever their limit is), they throttle the speed.

So check your ISP's terms of service and read the itty bitty fine print buried way down deep and you'll probably find this disclosed.

Comments

[Tim_Stewart]

Try downloading one of these files via torrent client. Make sure you port forward properly. Enable encryption on upload & download. Find something well seeded like a popular Linux distro. And see if the speed issue is replicated. If your using a browser or HTTP/HTTPS you are at the whim of the dl mirrors traffic shaping policy, or whatever bandwidth the host of said file has at the time.

[davidgo]

I largely agree with this - except for the bit where ISPs disclose this in their fineprint.

[Zackary]

In the Terms of Service, it says: CenturyLink makes no warranties: (a) regarding the security, accuracy, reliability, timeliness and performance of the Site or CenturyLink Connected Home service. But I find it weird that sometimes large files download full speed and sometimes don't.

[Windows11]

@Zackary pretty much each ISP is overselling, so you might download at full speed when there are less active users