This issue happens on all of my devices no matter what operating system is on it. Apps launch slowly, however the resources monitor says that CPU and RAM are not busy that much.
For instance, I installed a resources monitor floating widget on my Sony Xperia M2. What I found out that before I tap on a big app like FaceBook, the CPU usage is 3% and there are 370MB free RAM available. Then when I tap on the FaceBook app, the CPU usage started to vary within the range of 50~60% and the free RAM was changing within the range 300~350MB. However the app was still laggy, unlike higher-end Sony phone like Xperia Z2.
Briefly, the question is, why my phone is not functioning as fast as a higher-end phone of the same OS and brand even though it has 40% free CPU and 300MB of free RAM, why isn't it using 80 or 90% of the processor and the same in RAM?