With a heavily modded minecraft, this one creates a serius lag problem to the internal server (playing in singleplayer)

@rjulib00 without the logs we are unable to help, so please provide the logfiles if this is still a problem.

please show some proof, like log output, or profiling with VisualVM or atleast use /forge tps and /debug start/stop (over atleast 1 minute, the longer the better the results)

Also what version? (Futurepack, Forge, Minecraft)