Question Slow download speed and PC is lagging slightly when it's downloading big files.


Oct 29, 2014
Hello, I have a 150/150 fibre line, connected to my PC through a ethernet cable. Getting full speeds have been no problem since I got the fibre installed, apart from one time where this issue did happen, but then it just kinda resolved itself somehow.

The issue I'm having is that my PC slows down a lot when it's downloading files. I used Steam as a test since that tends to use the whole bandwidth. I get 3.7 mb/s at most where I should have closer 18 mb/s (and I have had that when downloading from Steam before), and the PC starts lagging quite a bit. I also did a few speed tests through Speedtest, and the results were 3 ping, 49.62 download and 160 upload. Usually it's at 1 ping, 160 download and 160 upload. As it should be.

My ssd and hdds are not full or heavily fragmented. Looking at the performance in task manager it barely hits 20% activity when it's downloading something. It doesn't matter which drive is the target, the issue still appears. Looking at the other activity monitors nothing is being overloaded. CPU, RAM, GPU, DRIVES all of them are fine.

I've also done speed tests on my phone where I get full speeds no problem, so the issue is somewhere in my PC. I don't know if it's a faulty internet adapter, or if it's something else throttling it. I can also confirm that this issue doesn't happen if I cap the speed much lower to like 10 mbps. It's almost like my PC can't handle the speed it's downloading at so it's throttling the whole thing, which makes no sense since my PC is more than powerful enough to download stuff at full speeds.

I've tried reinstalling my network drivers, running optimizer programs where it gets rid of unnecessary files and fixed registry issues etc, the same stuff I did last time I had this problem, but this time it just didn't help.

This is getting very frustrating, and I would appreciate any suggestions you guys would have cause I'm kinda lost on what to do.

PC Specs:
Operating System
Windows 10 Home 64-bit
Intel Core i7 @ 3.70GHz
Kaby Lake 14nm Technology
Micro-Star International Co. Ltd. Z370 GAMING M5 (MS-7B58) (U3E1)
ROG PG348Q (3440x1440@100Hz)
VG248 (1920x1080@144Hz)
3071MB NVIDIA GeForce GTX 1080 Ti (MSI)
232GB Samsung SSD 840 EVO 250GB (SSD)
279GB Western Digital WDC WD3000HLFS-01G6U1 (SATA)
931GB Western Digital WDC WD10EARS-00Y5B1 (SATA)
1863GB Seagate ST2000DX001-1NS164 (SATA)
1863GB TOSHIBA External USB 3.0 USB Device (USB (SATA))
Optical Drives
Sound Blaster Z
Network Adapter
Killer E2500 Gigabit Ethernet Controller

Thanks for any help!


Oct 29, 2014
" connected to my PC through a ethernet cable"

Have you tried a couple of other known working (at speed) Ethernet cables?
Yeah, I tried with the one that goes into my PS4, but the issue was still present.

However, when I booted up my PC today the issue was gone for a while, but returned 4-5 hours into the day. Kinda feels like it's software related somehow? I can't figure out what's causing it tho. I'll keep digging to see what I can find.