I've reported this problem to Support for almost as long as the change to the UI of the playlist editing function has been in place.
Historically, tracks had checkboxes beside them which made it easy to select multiple tracks to delete at once. After the UI was updated, tracks were able to be deleted by swiping them to the left or the right. In theory, this sounds easy enough. However, depending on the size of a playlist, swiping tracks to delete them or changing their position in the playlist can cause the app to lag severely or crash. When the app crashes, a notification explaining that the app is not responding appears but it can always be dismissed and the app used as normal for a few seconds until the notification appears again. If an app crashes then it should just crash. It should not be able to be used for as long as the notification that the app is not responding can be dismissed. The only way to resolve this problem is to force stop the app.
Related to this problem is how long tracks take to load when the editor is opened. Smaller playlists load all of the tracks almost instantly whilst larger playlists take up to a minute to do so. Once the tracks are loaded, scrolling through the tracks in a smaller playlist is smooth whilst in a larger playlist it is far from smooth and lags considerably. Swiping a track to delete it is smooth in a smaller playlist yet the red bar shown during deletion can stay visible and the track not deleted for up to a minute in larger playlists. It seems that since day one of this UI update, it has not been fit for purpose and that fixing whatever is causing these problems is of low priority to the developers.
Support has explained to me that they are aware of these problems and they typically happen when playlists of more than 100 tracks are edited. This is not the case as playlists of more than 500 tracks can be edited without the app crashing. Although, the larger the playlist, the more the app will lag. Last week, removing three tracks and changing the position of three others in a playlist of more than 1,100 tracks took almost 30 minutes. Once again, it seems that Support are aware of many problems yet hide behind the excuse of having a small team of developers meaning that problems take a while to resolve. However, once the problems are posted in this community for all to see, fixes to these problems are mysteriously found quite quickly. All I can say to Support is to stop playing games and sort out problems when they're initially raised.