The CPU is the controller for the memory speed -- what processor model is installed in the system?  
I recently upgraded to a DDR3L 8GB Single 1600MHz PC3-12800 module, and while it did all four passes of Memtest fine I noticed the BIOS, which is version A02 is reporting it as 1066 MHz. In one published Dell doc online it reports the speed as 1066-1333, (but also said ram is limited to 2gb or 4gb) but the BIOS has that setting (and many others) grayed out and locked so that I can't try any different settings. Is this underperforming and am I stuck unless I get a 1333 module, which seems to be more expensive? Personally I would be open to trying third party firmware even, though I kind of doubt it exists in this case. I'm a Linux user so am fine with open source. The Performance tab in Windows Task Manager shows 1600 but I'm afraid that may be just the maximum and not what it is actually running at, which I understand is what Memtest is displaying when it shows 1600. Do I have to do a benchmark?  Edit: I see even the module on the Dell parts page is 1600.
Inspiron 3135 BIOS memory speed