Why reduced while copying to pendrive?  

From the start copying fast a few minutes later its very slow copying.
Dont know why..
8GB pendrive (Empty) and in NTFS format.
My Laptop has 8GB ram !

Could be a number of things:
- the pen drive read/write speed,
- Defender may be scanning it en route,
- perhaps Trackers are being written to the pen drive as well...

Nothing wrong with those speeds.

A standard pendrive is quite slow. Particularly as I guess it is a rather old 8 GB one, and it appears to be 1 GB files.
You will find out if you do a lot of very small files it will be a lot slower than that 6 MB/s.

I have an old 4 GB Kingston pendrive, under test it is 6.82 MB/s with 3 x 1 GB + 1 x 760 MB files.

There is always that initial peak, it means nothing.