Just forcefeeding the game more RAM isn't always the solution. When I continued playing after a longer break, I often didn't even have 20fps in 300 battle size fights. First thing I noticed when looking for clues was the NVidia control panel had set the global selection of the GPU to auto, so the game apparently said "Eeeyy, your i7-4710HQ has Integrated Graphics!". Thus, my GTX860M twiddled its thumbs with a gentle smile while the CPU sprinted to 95°C. After changing the setting to my dedicated GPU didn't help that much, I opened 'er up (MSI "gaming" laptop) and well ... the interior and the thermal paste looked about as good as you'd expect it to look if it hasn't been cleaned in almost six years.
Cleaned the cooler, cleaned the interior, replaced thermal paste on CPU and GPU and voila, the game runs at 60fps at 60-75°C with 4GB of RAM allocated.
However, I'm only using LazyLib, GraphicsLib, Ships and Weapons Pack and DIYPlanets. And size 500 battles, where both sides deploy loads of fighters, still cause fps drop.