On starting shows 156kb/s on downloading 80Mb file,but speed suddenly reach to 5kb/s after downloading 3 Mb?

Sounds like your IP is throttling your speeds. Do you have a download limit as part of your internet package?

You got an interesting type of mail of 10 mails can make 7Mbyte up.

The speed on the internet depends on many factors, if the server you download from is "close" to you. How many hops it goes through and well of course the time.

The first burst can easily be the first buffer. Yup there are buffers everywhere and they can release their data in one push but obviously the actual stream behind it might be small.

Now 5kb/s is terrible slow for most use. So better call your provider for a line check!

Ask the sender to save the file as a Excel binary workbook (.xlsb format). It is quick.