I tried increasing the averaging period, but that didn’t seem to help. But the replacement of Fluid-O-Meter with Bucket-O-Meter helped. The Bucket-O-Meter does not track every liter of liquid and therefore changes the displayed value much less frequently.
I’m pretty sure that all the problems other than the one I pointed out in the first post were caused by the constant updating of the Fluid-O-Meter display. I set up two barrels, put a Fluid-O-Meter on them, filled one of them and connected them so that the liquid constantly flows from one to the other and this causes Chunk updates and stutters. Although the strength of stuttering can vary. If there is nothing else near this building, then stuttering is almost not noticeable. But when I put this building in the chunk that contains the reactor and a lot of other things, the stuttering becomes terrible. Stuttering and Chunk updates do not occur if the Fluid-O-Meter is not in display mode. If I understand correctly, the world is divided into cubes of size 16x16x16, and if something changes in the cube, Minecraft redraws this cube entirely. And the strength of stuttering during redrawing seems to depend on the number of objects in the cube, and possibly on the shape of these objects. Therefore, I think that if there is an object in the cube that regularly calls Chunk updates, such as a critical reactor, then it is worth reducing the number of objects in this cube to a minimum.
There is a way that partially eliminates stuttering caused by Chunk updates. To do this, you need to install ArchaicFix and enable occlusion culling in archaicfix.cfg. In this case, the stuttering disappears if you don’t see the object that calls Chunk updates (this is a simplified description, the culling algorithm is more complicated and I don’t fully understand it). This can be useful in the case of a critical reactor, as it calls Chunk updates regularly.
But there seems to be a compatibility issue between GT6 and ArchaicFix. When I fly over the reactor or rotate the camera quickly while near the reactor, I see stuttering even when the reactor is turned off. Stuttering is reduced by turning off the Phosphor lighting engine in archaicfix.cfg.