Hi Anne,
the V-bit has only a tiny working tip, so it cannot remove very much material in one go. The 1mm flat end mill can remove way more material in one go, so it is much faster. However, the 1mm flat end mill is unable to produce the details the finer bit can do. The typical approach in CNC milling is to do a “roughing path” with a coarse bit (like the 1mm flat end mill) to remove much material fast, and then do a “finishing path” with the V-bit for getting the details. Sometimes you even do more pathes for clean surfaces etc.
That said, Luban is very limited in its abilities for multi-path milling, but Skreelink put together a guide: Multi-Pass Rotary Guide
If you want to do advanced stuff, you’ll need to invest into a 4 axis capable CAM software. Unfortunately, these typically are really expensive…