Welding with projectors can cause very high sim impact
This started with the Scrap Pack release.
When using a projector with blueprint and shipwelder to build said projection, there can be a very large sim hit, easily bringing servers / single player worlds to their knee's. It seems to especially trigger after all blocks the welder can reach are welded and it's trying to check for other blocks that can be built.
This was a dottrace captured from a single player world with 2-3 grids, <20k pcu total in the world, no other entities during reproduction of the issue
It is difficult to reproduce in pure vanilla as the welder radius is so small; you have to line things up just right to make it occur. However if you modify welder SensorRadius to something larger (say 10 or 20 for example), it reproduces instantly.
Place either enough ShipWelders (or adjust their sensor radius larger) to build part of a blueprint
Start welding blueprint with projector set to "Keep Projection"
Either during the welding, or after the welding is done but the welders remain on, issue should present itself