Before importing, from within the iMovie app you can open an iMovie library that you have created on an external drive that is formatted Mac OS Extended (journaled). Then what you import into iMovie will go into the library on the external drive. Whether it makes editing faster or not depends upon how much data you have in the library. iMovie libraries that contain less data function a little faster than those that have a large amount of data. It's probably not very significant. It affects mostly the loading times.
Rather than importing directly into iMovie, the Apple recommended way is to import into the Photos app. You can then access your media from within iMovie by clicking on the Photos icon in the iMovie media sidebar. iMovie will refer to the photos and videos contained in the Photos app. Thus, the media is not stored in the iMovie Library until actually used. Keeping media in the Photos library rather than the iMovie library has advantages. The Photos app, being basically a management system with editing capabilities, makes it much easier to access, manage and organize files because you can use albums and search functions. Further, iMovie libraries can become corrupted, in which case you may have difficulty accessing and getting your media out of the iMovie system. Media stored in the Photos library is kept outside of the iMovie system until used in a project.
Some people on this forum import their files directly into a folder in the Finder, using the Image Capture app on the Mac. The folder can be stored in an external hard drive, and the files in it imported into iMovie as needed. File management is more easily done with media in a Finder folder rather than an iMovie library, because you have the capability of subfolders, plus the Mac's search functions. iMovie more of a movie editing app rather than a file management app.