I do love my Dell L521x, it’s worked really well for 2 years so far, but I hit my first problem with it after upgrading to Windows 10.
After the upgrade I found that I was unable to control the brightness of the LED screen in any way (either using the Fn+F4/F5 keys or via the controls within Windows).
The most obvious cause is a dodgy driver (the brightness controls would be disabled in Windows 8.1 if the Intel driver was installed instead of the Dell provided one). Therefore, I guessed that this must be the same issue. However, neither installing the Dell-specific driver not letting Windows Update put the latest Intel driver on there had any effect. Removing all drivers (reverting to the generic MS one) did enable brightness, but absolutely destroyed performance, so was unacceptable.
The solution was found on the Dell community support website, courtesy of a user named Drop76.
As strange as it sounds, disabling the UEFI secure boot in the BIOS fixed the problem. Everything worked perfectly after a reboot.
If anyone can explain why this should be the case then please let me know!