Hi @Kim_Hakseon,
Per the documentation page you linked, all index builds in MongoDB 4.2+ use an optimised build path and there is no longer an option for foreground or background builds. The background index build option will be ignored if specified.
The new index build process does not have the extensive blocking behaviour that was a concern for foreground index builds in previous server releases.
Index build performance in 4.2+ should be similar (or better) than before depending on whether the collection is being actively updated while the index is being built:
The optimized index build performance is at least on par with background index builds. For workloads with few or no updates received during the build process, optimized index builds can be as fast as a foreground index build on that same data.
Regards,
Stennie