https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1082209554

It's down to your:
* Storage device
* CPU
* Anti-virus or other kernel-level drivers
* ISP

Pick one (or multiple).

Steam downloads and installs at the same time.

1. Downloading the encrypted and compressed chunk (Internet)
2. Decrypting and unpacking the chunk (Disk)
3. Applying the diff delta to a new copy of the patched file (Disk) [if it's a patch]
4. Deleting the previous now outdated copy of the file (Disk) [if it's a patch]

Either usage limits the other: Slower internet slows down disk progress, slow disk progress slows down chunk download.

thanks for that ..very helpful,ill have a play around

One other thing to try that works for some people if you're using a SSD. Turn off write caching for the drive when downloading games on Steam.