This is indeed the mod I was using, and gave the name of it to wolfy.
This mod is purely client-side, and capturing, and loading schematics should cause no server lag whatsoever.
The only possible problem is with the print function. This function basically takes over your character to place blocks around you. In the mod options there are a few settings on placement distance, delat, timeout, whether it destroys blocks etc. If these settings are too OP, for instance if you have “Place instantly” true, or if your timeout is below about 15-20, then you are placing more blocks than the server can handle at once. If you are allowed back on, make sure to turn those specs down, or else the same thing will happen, and no-one will be able to place any blocks at all.
Also if the server is already struggling with lag, e.g. everyone is complaining about ping time, I would suggest just building it yourself from the schematic.
The mod was checked with admin on, I had John watch the lag. Even with instant on, it did not create lag.