I too have noticed quite a slowdown with large fleets in battles. Much more so than in Diplomacy (can't speak for the previous beta as i didn't have it). I've paid careful attention to my graphics card stats, memory stats, and processor stats and i can say with a fair bit of certainty it's not the video card or the memory causing the bottleneck. Video card is rendering at ~60 fps and running below high load temps, memory is steady at about 1.5 Gb usage, with a rather small pagefile. My CPU, however, is pegging solidly at 13% (quad core, HT, so "8" cores), which means it's maxing out one core steadily. During large fleet battles that would not cause diplomacy to slow donw significantly, i'm presented with extreme slowdown. Oh how i wish game multithreading was a thing by now. 
