It can be anything. Overfloved pool (distance fields, Nanite, Textures, loading of new assets). The same goes for culling or the blueprint needs to be adjusted. Sometimes it is necessary to profile it.
You can try any stats for lags, whether it’s large disk resources, memory, culling, etc.:
edit: Sorry. Screenshot was rescaled. At the top, find “unit” or “memory” in the stats. At top is “tools” and bottom is “derived data cache”. But its different situation in game build and in editor.