In #136061,

is mentioned as an area of code with quadratic complexity, which may lead to performance issues; though this is unlikely to be of large impact in practice because attackers with the ability to get a huge file loaded and/or modify configuration files to change the tab width can do much worse things than getting IDLE to hang on hitting backspace.

However, the complexity there comes from a loop which handles situations (files with tabs) that either cannot or should not occur; in the current default IDLE, you can't put tabs, and a space after a tab should just be deleted in a file coming from elsewhere as in indicated in #134874 (comment). Therefore, simplifying the function specification and/or deprecating IDLE to handle tabs (which we should do) will allow elimination of this piece of obsolete code, as indicated in #136061 (comment).