We've taken great pains to optimize Odin as much as possible. We emit code rather than using vanilla reflection, use GUI over GUILayout where ever possible. We're careful not to do more than is necessary to get the job done. The inspector does, of course, suffer a small performance hit in order to offer you more features; But nothing that we've been able to notice on our rather weak laptops.
However, when it comes to objects containing lists with thousands of elements, Odin currently suffers. We are planning on reworking how our property tree works internally in the near future, and one of the main reasons for this rework is performance. The main thing that is currently causing the lag when it comes to "big data", is Odin generating and updating its property tree. And this is where we plan to improve performance. Right now, we are currently updating all properties in our property tree each frame, regardless of whether you are inspecting the property or not, (List elements are also properties in our property tree, so there can be thousands). In our upcoming rework, we will only perform an update on those properties actually drawn in the inspector or marked as dirty.
If you experience performance problems with objects containing a lot of data, your best bet is to disable Odin for that particular type. You can do that from the Editor Types window found in Tools > Odin Inspector > Preferences > Editor Types
But we can only get the performance down as much as Unity allows us to. Unity is for some reason serializing the object you are inspecting multiple-times per frame, and that's nothing we have any control over. Because of this, the inspector will appear laggy regardless of whether you are using Odin or not when you are inspecting objects containing a lot of data.