System Information
Operating system: Windows-10-10.0.10240 64 Bits
Graphics card: GeForce GT 710/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 390.65

Blender Version
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-29 14:47, hash: rBf6cb5f54494e
Worked: (optional)

Short description of error
The playback speed or the frame rate (fps) is very low. The scene has some objects with armatures, materials, and particle systems. It happens even after doing all the possible techniques like hiding unnecessary objects and hiding subdivision surface modifiers. Rendering the viewport may be useful but not practical.

Why is this an optimization issue and not an issue with the hardware?
I opened the slow scene and I opened a very lite scene (default cube) in another window at the same time. The two windows are on the same screen. I played them both. The lite scene worked properly (24 fps).

If the slow scene was using the full power of my PC, the lite scene shouldn't work properly. It seems that the lite scene used the rest of the power that the slow scene didn't use. I googled and I found out that this issue happens with some sophisticated computers as well.

Exact steps for others to reproduce the error
You can simply open or make any complex scene with some big objects, moving characters, particle systems, ...etc. The FPS will dramatically drop to 2 ~ 4 fps. While playing this complex scene, open a new and play a lite scene and it'll play normally.

I know that the blender team and developers are very smart and can invent something that makes real-time playback (just like the great real-time EEVEE). It will be a great optimization.