Rendering can be frustrating so here are a few things that i found:
These are your default render settings. So first off, i disable “Reflective Caustics” and “Refractive Caustics” . Also i change the Transparency max to 6 , min to 2. Bounces max to 6, min to 2. Lastly Transmission from 12 to 4.
This is a simple one but in your light sources properties, enable the “multiple importance”.
You can also reduce the max bounces depending upon your scene i suppose.
GPU or CPU
Usually i would say go with your GPU but in some cases (physics simulations mostly) you have to render with CPU. Now in this case your Tile Size greatly effects your render time.
If i’m using GPU then i choose a tile size of 256 by 256. whereas if i’m rendering with CPU i go with 32 by 32 or sometimes 16 by 16.
You can also use the Auto Tile Add-on. Enable it in user preferences and it automatically calculates the optimal tile size.
By just these you can cut down your time by more than 60%. It can save you HOURS of rendering. Still learning.