Hello,
1) With [High-Speed mode], you can quickly cut/merge videos without converting (encoding) them and without decreasing the quality.
2) If you can't use [High-Speed mode], you must use [Encoding mode] of Bandicut.
In Bandicut Encoding mode, if you set the file size to the "original size"(#5 of the image below) and select the quality to 100(#4 of the image below), you can get a better quality video.