On Forge Island, the primary source of framerate drops is simply object density. Any object that is even partially visible on screen at any given time must be rendered in its entirety. If the game renders too many objects at once, framerate will hitch. Any part of a map where too many objects are visible will potentially be subject to frame drops. Even if the area does not cause frame drops under normal circumstances, the addition of players, projectiles, explosions, etc. in an area where many objects are visible will often tip things over the edge and cause framerate to hitch. There are some objects which are more difficult to render than others, but the variation on Forge Island is minimal and you will find problems coming from overall object density rather than the use of specific objects.
However, the other forge canvases are subject to more complex issues. Impact and Erosion are particularly susceptible to frame drops, meaning that relatively lower object density can still cause frame drops on these canvases compared to Forge Island. The items available on these canvases as well as Ravine also include more variation in regards to how taxing they are to render. Station corridors, silos, and other canvas-specific items are often more likely to cause frame drops than simpler items such as blocks and bridges. Lights and special FX as well as proximity to specific landmarks (the light posts on Impact, for example) as well as numerous other factors can contribute heavily to framerate loss. It is important to remember that no object on its own will cause framerate issues, but when used in large quantities, any object will contribute to frame drops.
Lastly, the thread linked above details objects' effects on the game's theoretical lighting budget. The values given in that guide do not directly translate to the effects of these objects on framerate performance.
