I like to do large galaxy long term games but they don't work very well in Master of Orion: Conquer the Stars.
After 600 odd turns, the save game files begin to exceed a gigabyte (yes a gigabyte) in size - taking a long time to save (long enough for Windows to declare the game "not responding" for a minute or two) and load. After 800 odd turns, the save game files are nearly doubled. I can't think of a reason why the save files should possibly be this large.
Also, AI player turns and analysis during diplomacy begin taking a very long time. Even just proposing a technology trade can take 2 minutes for a response on a high end system (i7-6700 3.8Ghz, 16GB RAM).
Unfortunately, right now I don't consider the game playable with large scale games in the current state.