The PDF documentation included in the downloads has the full details and images as well as updates on the latest features. This is a small overview of the most required informations.
How to use the speed-up?
Open your file and render, as simple as that. For more than the base speed-up (about 2x faster), you can select a profile like "fast" or "very fast" (2 clicks at the top of the quick settings panel). Using the AI denoiser also allows to lower the sample count by 4 in many cases, giving another 4x boost on top.
How to use the persistent data option and what does it do?
In the performance panel, the persistent image option was replaced by persistent data. In scenes where only the camera moves through the scene, this option can dramatically speedup your overall render time by only doing the pre-processing step once for the whole animation. Particularly useful in complex scene with long pre-processing times.
As you can see, with persistent data in this scene, the overall render time after the first frame is 4x faster with E-Cycles than with the buildbots.
What if I want to use E-Cycles with CPU or OpenCL?
E-Cycles is 100% compatible with Blender, so it will work, but the base speed-up is only for CUDA and OptiX. All other features will work on CPU and OpenCL. The standard version of E-Cycles has everything you need in this case.
Tips to increase render speed:
minimize the Blender window when rendering, it can make rendering much faster.
if you have multiple GPUs, prefer using the AI denoiser instead of the included in Blender. Blender's denoiser currently has a bug which makes it very slow with multi-GPU setups.
in the user preferences → system, disable your CPU in CUDA. Only if your CPU is much faster than your GPU (for example threadripper with mid range GPU) and you render at high sample count will it be better with CPU and GPU. All other cases (low spp with denoising, high end GPU, etc.) will be better with only your GPU(s) activated.