I can't edit my previous post which is pending approval (joined in 2013, never ever posted) so sorry for the double post. I managed to mitigate a lot of the slowness by deleting everything in the SquadExpansion folder, which freed something like 1/5th of KSP's memory use at idle and somehow also slashed the loading times in half going between the editor, KSC and flight scenes. I figure none of their assets were going to be used by RP-1, was I wrong?
I also deleted a few leftover part folders from an old version of KSP that for some reason Steam had kept all along, namely the standalone T200/T400/T800 fuel tank parts from 2017-18, since replaced by those from the Size1_Tanks folder. It solved a bug I encountered when trying out the corresponding parts, they seemed to work fine at first but whenever you would load back the vessel, all the parts would be clipping. In case anyone runs into it too, there is your fix, I haven't seen it mentioned elsewhere.
04/04 edit:
I can't figure out the intended way to use procedural tanks from Procedural Parts as in-game descriptions contradict each other. I understand that the "Tank I/II/..." series of parts is deprecated and all descriptions point towards the "Procedural Tank" part, which seems the most up to date and does switch between various available technology levels ; however, this part is also marked as deprecated and its description simply states "Do not use this part."
What is the logic behind it and what should players use, then? Modular tanks from ROTanks are a nice port of SSTU and do look more realistic, but I still really like the variety of Procedural Parts tanks in their varied textures, the shapes you can get out of them (cylinders are a bit restrictive) and vastly prefer their sizing in meters rather than playing with part variants and scale like on the Modular tank.
On another note, stock 1.25m tanks (unlocked at Post-War Materials Science and seemingly the only unlock of this node, bar radial and toroidal tanks from PP) are a bit broken in their mass ratios relatively speaking. I haven't unlocked type III or IV tanks so I can't compare, but stock tanks beat both type I and II (the latter unlocked at the following node, Early Materials Science) in mass ratio, on top of being dirt cheap and not requiring tooling... or fiddling to get each new part to exactly 1.25000m diameter.
Small comparison :
A stock 1.25m 1kL T200 tank set to "Default" costs 5 credits, is about 1.15m long and, when containing a full load of A-4 propellant mix (incl 507.6 units of Ethanol 75) has a dry weight of 32.12kg.
A Procedural Parts cylindrical 1.25m tank with similar 1kL capacity in "Tank-II" configuration costs 9 credits tooled, is 0.926m long and has a dry mass of 52.73kg (again, when loaded with A-4 mix).
A similar tank from ROTanks costs 11 credits tooled, is 1.336m long and has a dry mass of 53.25kg.
The comparison goes similarly between a procedural Service module (SM-I) and a T-200 configured as Service module.
I hope this is not considered spamming, I merely wish to help iron out the issues that I stumble upon, from a relatively fresh perspective.