SevTech: Ages 1.12.2. Server ip: sevtech.minewonderland.com Modpack version: 3.0.8. Server features: economy; global ranking; land claims; market shop (/market).
You should also list your system specs just to be sure. This is a very heavy weight pack, even for big packs standards.
1: Are you allocating enough RAM to minecraft? For this pack it seems that 6 GB is the bare minimum you need to allocate. I would personally recommend at least 8 GB.
2: Are you adding extra recommended java arguments for garbage collection?
-XX:+UseG1GC -Dsun.rmi.dgc.server.gcInterval=2147483646 -XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
-XX:+UseG1GC -Dsun.rmi.dgc.server.gcInterval=2147483646 -XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
3: Have you tried adding Optifine to the modpack? It can greatly help with performance.
4: Are you keeping open twitch livestreams while playing? Livestreams are CPU intensive, especially if you're playing them in 1080p60, and Minecraft loves to eat your CPU.
Performance issues need more information than just: 'The game lags what can I do'. You need to give plenty of information because, contrary to consoles, every single computer is different and will have different performance