I just installed onto my Dell Latitude E6520 and the network speed on both ethernet and my WiFi is unbearably slow.
I have Gigabit internet and the connections were both fine with Windows 10, Steam is currently downloading a game at 100-500kb/s and webpages are taking over 5 seconds to load.
The wireless card I am using is the Intel Centrino Advanced-N + WiMAX 6250 since the broadcom one I had before was garbage.
Hello IndustryStandard
I'm no expert with regard to these matters, but you might try the "fix" given in this post:
And if that doesn't help, this may be of use:
https://wireless.wiki.kernel.org/en/users/drivers/iwlwifi
Let us know how you get on. Good luck. 
That made it worse unfortunately.
Hello IndustryStandard
This post tells us that everything should be working well for you...
[https://www.reddit.com/r/linux/comments/7inpj8/dell_latitude_e6520_seriously_underrated_linux/], but it is not (yet). As I said, I'm no expert but I found some more information that might be helpful:
(A)
BIOS - have you gone through all your BIOS settings to ensure that nothing there is "switched off"?
(B)
BIOS - did you install with a Legacy or UEFI (Unified Extensible Firmware Interface) BIOS setup. Changing the BIOS mode and re-installing might help - or not.
(C) - identify your PCI Ethernet adapter, open a terminal using the keys CTRL+Alt+T (you can close the terminal by typing "exit"). Enter the following:
lspci -nn | grep Eth
This shoul produce something like this:
00:19.0 Ethernet controller [0200]: Intel Corporation Device [8086:1502] (rev 04)
Do an internet search for "8086:1502", see what you can find out. You might find something useful.
(D) - another distro? This post suggests that openSUSE might work well (the reddit post indicates that Debian works too), with regard to the networking problem. But if it's a question of drivers/other things being in the kernel Ubuntu-Mate should also work.
[http://www.linlap.com/dell_latitude_e6520]
(E)
This is a little more involved, you may or may not want to try it:
[http://giantdorks.org/alain/fix-for-non-working-wired-ethernet-on-dell-latitude-e6520-with-intel-82579-based-adapter-running-ubuntu-10-04-lts-lucid/]
Let us know how you get on - but if you need more help one of the wise and knowledgeable will be required.