I´m working with 36 MXF project and when importing material there is something new in Source browser. What is the difference between uncompressed and RLE  alpha encoding? Do I save time if I choose RLE? How does it impact the working process later?

assuming you are using 2018.9........   You can now import, render, transcode and mixdown at uncompressed quality. This allows you to generate media that preserves the full quality of the Timeline (currently the Timeline is 16 bits maximum). DNxUncompressed is available at anyraster dimensions (HD, 2K, UHD etc.).

Starting with 2018.4, you have the option to choose to import an alpha channel as either Uncompressed or as Run Length Endcoded (RLE) which is used to reduce storage size and bandwidth during playback. (RLE) is a form of lossless data compression where runs of data are stored as a single data value and count. The default is Uncompressed. Access the Project Window Import Settings to select RLE compression.

this is a good place to educate on uncompressed versus RLE.

https://www.fileformat.info/mirror/egff/ch09_03.htm

Marianna