The CurseForge Launcher does not have the default JAVA Arguments found in default Minecraft launcher.
Not having any JAVA Arguments causes the JAVA garbage collection (memory cleanup) to cause client lag such as judders or client lockups, commonly known as a lag spike. To fix this CurseForge lag, follow this guide:
1. Open the CurseForge app
2. Click the Gear Icon in the bottom left
3. In Settings, under Game Specific click Minecraft
4. Scroll down to Java Settings
5. In the Additional Arguments box add
-XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
6. Click the Cross Icon in the top-right to exit settings
7. Close any Minecraft Launchers you have open and re-launch the modpack
Everyone who plays on the server will need to do this, share it with your community.