I've been running both unmodded and a modded servers on my computer while playing Minecraft. When I have a vanilla server running I can notice minor lag spikes or stutters about 2 times a second. I've installed Optifine to my game and that seems to hide the issue. However when I run a modpack server, the stuttering reappears and is much worse. Stutters still happen about 2 times a second but is very noticable and affects my gameplay. If I turn off the server the stuttering disappears and the game runs smooth as butter. No matter how much or least amount of ram I give the server it still stutters. While running the modpack server I notice stuttering outside of the game, like browsing the internet.