Assembly Analyst allows you to select certain parts of your factory for analysis, providing you with a breakdown of what your assemblers, miners, labs, and inserters spend their time on. This includes being active, having their output full or their input empty, being low on power, or being disabled. This shows you where your production is bottlenecked, and for what reason a machine isn't active.
To select a part of your factory, the mod add a Shortcut bar shortcut, which might need to be enabled first. To remove an analysis zone after you don't need it anymore, shift-drag over it with the Quickbar shortcut selection tool in hand.
This is, at a basic level, what Bottleneck does. What Assembly Analyst provides is a more detailed view of the status of your machines, and more importantly, a history of the machines' status. This allows you to notice, for example, that most of your gear assemblers only spend 70% of their time actually producing, and are idle for 30% of the time because their inserters can't fill them with iron fast enough.
The mod is optimized for performance, but that doesn't mean that it is super light on UPS. It's really intended to analyse local production lines after you built them. You're not meant to leave the analysis zones active permanently, and you should try to limit the size of them so your performance isn't impacted too much.
Assembly Analyst works with any content mods, it's fully multiplayer compatible, and can be added to a save at any time. If you have any suggestions, or encounter any bugs, I would love it if you told me about them here on the mod portal (under Discussion). You can support me on my ko-fi page, linked below.