I think this has more to do with this system not being tested with Windows 10 by Dell (limited drivers/support), and because of this preventing the best function of the nic. It might be worth searching for 3rd party drivers/solutions on the Broadcom with Windows 10, however if that doesn't work an easier solution would probably be installing a gigabit ethernet card (probably easiest to use an Intel one in this case, just make sure you have the proper pci-e slot). As for the wireless card it would need to be used on the 5Ghz frequency but because of this it will limit the range and doesn't travel well through walls (I believe this system shipped with the Dell Wireless1525 which should have 5Ghz). Under perfect conditions you might be able to get close to 300 mbit, but with a desktop a hardwired gigabit card would be the best bet to get consistent speeds since they're sometimes stored out of sight. I hope this helps, and definitely believe a gigabit Intel card will be the easiest fix, assuming nothing else looks strange in the Device Manager. Best regards, Brad 
Hi, I just upgraded our Cable Internet from 64Mbps down 5Mbps up to 400Mbps down 20Mbps up. As well as the Inspiron 580 MT We have several Macs here plus iPads, phones etc. All the Macs hit the 400Mbps mark on test, also if switched to Windows 10 via Bootcamp, but the Dell can do no more than 120Mbps, irrespective of whether it is on Ethernet (Or Local Area Connection, as Win 10 now calls it!) or Wireless. I have also checked over several Days at different times. Looking at the drivers, they are all up to date. We have upgraded over time to a Seagate Hybrid drive, 8 GB of Ram, & an Nvidia GT220 Video Card. All have been in place for some time without problem, & I cannot see anyway how they would effect the Internet, but I mention them in the interests of "full disclosure" :emotion-1: Still has the original Intel Core I3 530 (2,93Ghz, 4MB) Processor  Have we just hit the internet speed limit on this 7+ year old beauty, or is there something I am missing? Open to all suggestions & possible solutions!   Thanks in advance!  
Inspiron 580 MT slow Internet on both Wired & Wireless