If you have a lag issue, it's best to make a .nps file that you can send to the developers so they can investigate further.
Sampler is a mod by Player that generates .nps files. You can download it from https://forum.industrial-craft.net/thread/10820.
For Minecraft 1.12
For Minecraft 1.11
For Minecraft 1.10
Then you can
/sampler start and
/sampler stop while it's lagging.
Then a simple
/sampler export [name of exported nps file here] will write out the .nps file that can be read by the developers in VisualVM.
Important: where to run Sampler?
Is it a lag issue on the server, like tick lag? Run Sampler on the server, NOT on your client!
Is it a lag issue on the client, like FPS lag? Run Sampler on the client, NOT on your server!