Hello to all 
I am using Debian Jessie 5.25 on OPi PC+. And using rpi-monitor for monitoring stuff. I already made a fully working webserver and moved my wordpress to it, CPU usage around 10-20% when taking traffic, so it works like a charm.
Today i decided i will use it to download a few big files with wget command. At first i was trying to be smart and save it directly to an ntfs pendrive mounted at usb0. Result - 40% I/O wait CPU usage and limited transfer (maybe 20-30 KB/s). Yes, my pendrive is bad, but hey, not so much.
So i cancelled wget and tried to download the same file into /var/ into my SD card (used as system disk). Well, transfer was ok (200kB/s server limit), but i still had around 25-30% I/O wait CPU usage. CPU total is around 40% but it doesn't do much because its pretty cold, i think it just waists a lot of time to save file into SD (my SD is not top shelf but its pretty decent, its 32GB class 10).
Simultaneously with this massive (200kB/s) wget download i tried other things:
Tried to move (mv) 1GB file from system SD to usb0 pendrive, it transferred only 10MB in 2 minutes.
Tried to move (WinSCP) 1GB file from system SD to usb0 pendrive, it timed out very quickly and threw connection error or something.
Tried to download (WinSCP) 1GB file from system SD to my PC, it started at 4MB/s and dropped to maybe 500b/s after a minute (yes, bytes!).
My average WinSCP - PC transfer is around 8MB/s in both directions... when wget is not downloading anything.
And i even stopped apache service to make sure it wont interfere with SD access (sql and stuff) - user CPU usage dropped to 0% but I/O wait is still at 25%.
My question is - is it normal? Is CPU so much busy while copying stuff? I don't thinks so, i was moving stuff between disks before and transfers was very decent. Or is it wget problem, maybe it tries to save data with very small chunks very often so storages can't keep up? I noticed it doesn't matter how fast download is, I/O wait is pretty the same for 500KB/s and 5MB/s.
Yes i know i have eMMC which is much faster, but i don't want to move my system to it, not right now. Unless i can mount it as a normal storage and run wget again?
Best,
mrm noob
/edit
So i was wrong. Its not the wget problem, its accessing usb pendrive problem. Wget is still running, but I/O wait suddenly dropped to 0%. I checked and i have one "test" file missing from /var/ and it can't be found nowhere - file i was moving to usb0 but cancelled it by ctrl+c, but it seems that mv command was still working in the background moving my file, and at the end it deleted it from both places?
I set another file to move from SD to usb0 with mv, it moves very slow, maybe 100-150KB/s and... uses 25% of I/O wait. Why? NTFS format or what? Tomorrow i will try with different pendrives.