Quick answer: Unity editor locking up during mass import of assets? Default import is single-threaded - enable Asset Pipeline V2 parallel import in Project Settings.
Imported 5000 textures; editor non-responsive for 20 minutes.
Enable parallel import
Project Settings > Editor > Asset Pipeline V2 > Use Parallel Import. Imports run on multiple workers.
Or import in batches
Drop assets in chunks of 100; import each batch; editor stays responsive between.
Pre-compute outside Unity
Texture compression and mesh optimization run faster outside the editor. Import processed assets; skip the editor work.
“Asset import is bounded by Unity's pipeline. Parallel scales; serial doesn't.”
For projects with large asset counts, parallel import is mandatory. The Settings change is free.