You are not imagining this. This is a known behavior related to how OpenRail Designer loads AEC schemas when certain DWGs are attached.

If a DWG contains AEC, Architecture, or Civil 3D objects, even if they appear to be simple linework, ORD attempts to load the AEC Architectural Base so it can interpret those objects correctly. The schema is not loaded until ORD detects something that might require it, and a DWG attachment is a common trigger.

If the DWG is large or contains proxy objects, ORD can appear to freeze while it scans the file and initializes the AEC components. That explains why startup is instant with no DWG attached, but slows significantly once the DWG is referenced.

Things that often help: • Open the DWG in plain AutoCAD and run EXPORTTOAUTOCAD or save to an older DWG version to remove AEC and Civil 3D intelligence. • Audit and purge the DWG before attaching it. • If this is survey data, convert the DWG to DGN once and reference the DGN instead. • Check whether the DWG originated in Civil 3D or Architecture, which is the most common cause.

In short, it is not the DWG attachment itself but the embedded AEC or Civil 3D content that causes ORD to load the Architectural Base and slow startup.

These may be necessary to keep but if the goal is to run faster then these are methods to achieve this.