Is that normal that wget app reports (during downloading) much lower speed than the speed I'm getting when I use a normal browser to download the same file?
This difference for some files is like 10 times: 200kB/s (wget) vs 2MB/s (Firefox).
There should be no meaningful difference. Did you try different servers? My first thought is that maybe the server is limiting your speed based on user agent or cookies.
I tried downloading http://ftp.funet.fi/pub/NetBSD/NetBSD-8.1/amd64/installation/cdrom/boot.iso
with Firefox and wget, both gave around the same speed of 6-7 MB/s. Wget however reports more frequently so I can see the fluctuations where it goes below 6MB/s and above 7MB/s momentarily.
How does wget calculate/report file download speed?
Does it show average speed since start of download or (just like in case of Firefox) does it show current temporary speed?
It is the current speed.
Maybe I should use some additional options to increase some wget's "default speed limit"?
There is no speed limit by default.
PS. Can I use wget to download only some part of the file?
I don't think there's support for range requests (except when using -c to continue a partial download).
See if curl could help you there. https://curl.haxx.se/download.html https://curl.haxx.se/docs/manpage.html#-r