For anyone who have read this before my edit and marked it as already read, I started this thread asking for hints on how to do what I finally figured out, so I changed title and content to include the procedure of reversing the town scene of Minas Tirith to its less demanding version.

I still hope that a mod developer will look at this thread and maybe will consider a solution to the problem like an optional patch to add less heavy assets to the mod release (or a mod release with light assets with the option to add heavy ones if wanted).

This mod not only its unique and changes the Warband gameplay a lot, in a very good way, but it has always been incredibly lightweight. When a player loads a scene and looks for the first time at Minas Tirith or at Vale, his first thought is "my God, my computer is going to explode", instead I achieve personally a stable v-synced output of 60 FPS. Care has been put on the scenes, I've seen, to make them light while cool looking. The walkable parts are limited but the other parts seem alive anyway, dolls have been used wherever possible to avoid adding too many NPC (one of the main causes of FPS drops). It would be a true shame if this incredible work of optimization of these leggendary assets would be jeopardized by new contents surely even more rich, but technically not very feasible.

I hope this little guide will help in while anyone with the same small issue.

P.S.
I'm still curious if anyone can achieve 60 FPS with the new Minas Tirith during a siege, and which GPU/CPU uses. In my case, the used threads of my i7 4790K (oc at 4,69GHz) were at about 80%, but my MSI NVidia GTX 980Ti Reference (oc at +180MHz both for core and memory) was used at 100% (so the bottleneck). Maybe that with a much more powerful gear, including a single thread powerful CPU (like the i9 12900 KF and an RTX 2080Ti or 3080Ti), these scenes wouldn't be a problem anymore?

Since I got no answer from mod authors, I posted a message in the comment section, but got deleted.
I decided, then, to create a guide, available at this link:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2767879013&searchtext=the+last+days

I truly hope that someone will reply to this plea.

Sorry for the late reply. I skimmed through this. Seems like you sorted out your problem yourself, good work.

I'm currently working on a new version of Minas Tirith that will be slightly more optimized, if that's any consolation to you. That said, I have a worse rig than yours and I'm getting 40-50 fps in a siege, depending where the fight is happening.

Also, Warband is 99% CPU limited. So if your GPU load is at 100%, that's probably something you'll want to investigate on your end.

Thank you for your answer, I'll make sure to double-check my components.
I'm looking forward for your new version of Minas Tirith, but I'm sure it will be beautiful as all the other scenes of the mods. I'd just never stress enough to keep in mind performances and optimization rather than putting too many decorations, or maybe to make available an optional patch for whoever has more powerful machines (or viceversa).
Of course, if the guide is of bother, let me know and in any moment I'll remove it.
Thanks again for the kind attention and keep up with the good work.

TLD is open source, we don't have any problems with people adjusting it to their likings.