I have a weird problem. After some hours of uptime, 4 to 8 hours generally, when I download a file to any place: memory (when Firefox starts download files before saving to disk), system's SSD, internal or external HDD or usb key; the computer slows down:
mouse andsound lag, fps drop/video acceleration problem, and so, until the file is completely downloaded.
The problem does not happen on Ubuntu or Arch Linux, both installed on another drive (HDD, not SSD). I didn't have this problem on Windows 8.1, installed on the HDD used now for Linux.
To make the problem disappear, I can restart the computer. Also, downgrading/installing the Ethernet card's driver sometimes stops the problem (=after some hours of uptime, it happens again). Just tried two hours ago and it didn't work, so it's like getting worse.
I don't think I have a virus, I don't install any unknown software, I prefer to use open source software that have communities. I use NoScript in my navigator and don't allow untrusted websites. I have also a paid anti-virus, Bitdefender (may cause an issue but I don't know how to disable it for a time like it's possible on other anti-virus - had it on 8.1 too) Is it safe to uninstall it to try? I haven't tried and I have to wait some hours to have the downloading problem...
My PC's specs (self mounted):
CPU: Intel i5 3590k
GPU: GTX 960 & GTX 760 (no SLI bridge, I bought a non working one, removed it), driver up-to-date from GeForce Experience (release version)
MB: MSI Z77A-G45
Integrated eth card: Realtek 8111E (driver up-to-date, from Realtek.com.tw when the websites is not slow, but does the same with the Windows' one)
SSD Trademark: Silicon Power (2,5 SATA III State Drive Slim S60)
OS: Windows 10 Pro, 64 bits
Please ask for any needed information, and thanks for your help.
EDIT: Forgot a thing:
When it happens, I download from Firefox or a command line tool, and the process named System and compressed memory uses 40-60% of the CPU and the first core is completely used. When I download from Filezilla (SFTP), it does the same but with the process named System interrupt (process names are translated from French and may not be the same in English language).
EDIT 2: Also, It's not a mouse/keyboard lag, it's FPS drop, because the mouse moves but the frames are not shown). But there is effectively a sound lag (like when you read slowly a sound but more buggy).
EDIT 3: I kept my computer on, and the problem appears and disappears about every 20 minutes, and has not always the same severity...
EDIT 4: Yes, I think it's caused by the anti-virus, because it has multiple "modes", and switching between them change the way the problem happens.