help!! download speeds half when using other programs (not fully saturating my nic)

[Valkyrie743]

Hey all.  my current internet is 500 down 20 up (spectrum)  currently running a older netgear gigabit router with DD-WRT.  running hardwired. 

i've noticed for a while that when im downloading anything from steam, or origin, blizzard doesn't matter i can get the full 500 down but if i have chrome open (while not having any network activity) or viewing files on my local nas. my download speeds drop (sometimes as much as half).  if i want to take full advantage of my internet speed while downloading. i have to close everything other than the program that is downloading.  ill watch task manager stay at a steady 500 down +/- 15 but once i open chome, or open explorer and view files off my nas, or anything network related it will drop to 200 +/- 100  

im only using half (50%) of the gigabit lan when i only have steam or whatever downloading open so why is it dropping my speeds?   i've checked my nic and its running at full duplex not half.  my system is i7 5820K on a asus x99 pro motherboard (intel gigabit nic) 32gb ram, windows 10. 

is there some sort of setting that windows has that throttles the nic so other programs that use the network they dont bog down from something else? how can i fix this so i can download at full speed and not have it drop when just browsing the net (not streaming video) and or viewing files on my nas?