Memory bus speed

The 4 in DDR4 means 4X -- the bus speed is 667, so multipy by 4 to get the 2666 the RAM is designed to run.

The system just isn't designed for gaming or heavy workloads.  That's the real issue - it's a budget system, not a performance one.

@ejn63​ thats not true, 4 is the generation. I have a PC with dual DDR3 1600 MHz (effective clock) and the bus is always at 800 MHz (so 1600 Mhz effective clock) because its DDR. 
In this system the BUS goes to 1330 and then goes down to 665.

DDR4 is bus speed X 2, and then in dual channel mode, X2 again -- i.e., 4X.  

The memory speed is not constant - it will also fluctuate with the system demands. 

There's nothing wrong here -- the system is running as designed.  No, you won't get the same performance out of it that you would of a mid-range or higher system, but you don't get the same performance out of a Hyundai Accent that you do out of a turbocharged N-line model, either.

@ejn63​ I'm not trying to get the same performance as a higher system. If what you're saying is true, then why the bus speed goes to 1330 MHz (FSB 40:3:) when system is in idle and then goes back to 665 MHz?

my pc has a dual ddr3 1600 MHz and bus is always at 800 MHz, if I change frequency in bios to 1330 then bus stays fixed at 665 MHz.

It may be you have applications that run when the load on the system is low, which causes the system to throttle up the bus speed.  When the load is lower, the speed will decrease.  Most of this is handled by driver and firmware on your notebook - it isn't manually adjustable as it is on a desktop system (particularly one built with an off the shelf rather than an OEM custom mainboard).

Desktop systems aren't anywhere near as power- and cooling-limited as notebooks, particularly notebooks like your 3520, which is designed to be as low cost as possible, which limits the materials used in the heatsink and other components.

@ejn63​ Yes, I got it. I hope dell release an update to solve this. I understand the BUS at 665 MHz when the system idle or having other problem like temperature (I'm using a cooler pad to help on this), but this is happening when I need better performance...