Another user reported this issue and I noticed it might be legit while updating the Voltz M12 modpack. There seems to be a decline in performance after a reactor is enabled. Not sure what the cause is, it could be the particle effects or the backend. I suspect the first but have not tested it fully.

Im having the same problem except its rather when I try to open anything it lags out then finally opens and this only happens while the reactor is on.
If I turn off particles however the problem stops

Its likely the initial generation of the heat and radiation grids. As well the spam of packets you get from the reactor and boiling effects. One of the solutions that is on my list is to add a startup delay. However, I also need to look at a pathing delay on top of this to slowly start the grid rather than to slam it into full motion on map load.

I was experimenting with my optifine settings like yesterday and I decided to change the resource pack to change it up and the lag just stopped, I still see particles but it doesn't lag at all so I don't know what's that about

Optifine might have triggered some optimization to how the particles are rendered. Most lag people run into is due to the particle spam that we currently use. The plan is to replace these eventually with a shader render that would do the particles on the GPU in one pass rather than doing them 1 at a time on the CPU then GPU.

why not force optifine to do the rendering for you at the current moment then? until you get all the shader coding in place

oh also I love the mod don't stop updating it :D