[Nexus 6P General] Spartan117H3:
I was saying that those who use roms that have issues are either using roms that do not have stability as their top priority, have bugs/are experiemental, or it's a user issue. The OP says 100% stock, he had root and an unlocked bootloader but says that it somehow degraded/lagged over time (which is absurd) so he went and locked it and went 100% stock. That is worlds apart from Stock + root + Xposed.

TL;DR, roms are usually made more specifically for a certain phone but Xposed is made to be 'executable' on 'all' phones. Roms provide higher stability, less compatibility issues, quicker developer support (YMMV), phone specific feature implementations, etc.

I used to use Xposed with GravityBox and Wanam on my Verizon Galaxy S5. Xposed is very powerful, but it is also very volatile. The purpose of GB and Wanam is to make features so they are able to blanket 'all' phones. This means that features may or may not work. On the other hand, if a rom is built correctly, these features should work. If not, they are fixed in the next revision. There are claims (that I have no support for) that Xposed + GravityBox/other mods use more battery compared to a custom rom. I can assume that since Xposed code is made to run on as many devices as possible, it needs extra code just to be able to work with phones that are more restrictive (i.e, GB adds 'special' compatibility for TouchWiz/Samsung phones). Custom roms should be more efficient because they only have the specific device to code for. If a person wants a custom theme engine (Layers works with Stock 6.0 but CMTE does not), Xposed does not cover that. Roms also allow for patches much sooner than they are released. I realize Nexus phones get the fastest updates from Google themselves, but developers can sometimes implement patches faster than Google wants to release them (CyanogenMod baked the 5.0 Mem leak fix into their rom/someone released an Xposed module to do this as well). Xposed is limited in what it can patch, whereas roms are not. Having patches like this integrated into the rom itself also saves less technically inclined users from searching, or figuring out what to search for.

Roms showcase features that users may not know about. A lot of xposed modules that I've come across over the years/me going through all of the modules manually. Before I saw them, I had absolutely no idea they existed. It's much easier to find a feature in a rom and use it than search for something/a modification that they may not even know exists.

I have heard much about Cataclysm, but there is also drama associated with it that I don't feel like getting involved in. I don't blame the developer and understand his position. But anyways, you said it's better than other roms (apart from Cataclysm) which is a bit of a contradiction. You have some reason to say that Cataclysm stands out above your current setup, why doesn't that answer your question that you posed to me?

TL;DR, Performance and battery life is extremely subjective. If you have Xposed, you obviously installed something more than if you were on stock without it, meaning you cannot compare the two. Not to mention, everyone's usage is different. I can agree that kernels that force more efficient behavior, Greenify forcing apps to hibernate when not used, Doze allowing very slow battery drain when the phone is not in use, all collectively contribute to power savings here and there. But what if I wanted to play a graphics/processor intensive game for 4 hours straight, and we are only looking at those 4 hours? There is no efficiency that you can do to lower the battery consumption because at that point, it becomes a hardware limitation. The same works for the other end, if I Doze for 4 hours straight, even if I killed every possible app imaginable and had no background apps running at all, I'd still be limited by the hardware's minimum idle drain.