If your computer is lagging on the standard version of Entropy 300, use version 300-Lite under the modpack options. Lite version doesn't have a couple of aesthetic mods and the graphics are turned down.
The new Glixyl's Entropy update to Minecraft 1.12.2 will break your old worlds. If you want to play those old worlds, revert to version 209 under modpack options in the launcher.