I use Comment Add/Remove of UltraEdit nearly daily. In all versions of UltraEdit Comment Remove is slow while Comment Add is very fast. A test with 1000 lines selected in a *.c file with UE v11.20b, UE v220.127.116.118 and UE v15.00.0.1046 on the same computer confirmed that.
I guess, for adding a comment UltraEdit uses internally a regular expression replace or Column - Insert Column, I don't really know. But for removing a line comment it analyzes line by line. I don't know the reason for that difference. So for removing line comments at start of hundreds of lines I use a regular expression replace in selected text, sometimes Column - Delete Column which is also fast, but requires that every line in the selection starts with a line comment. Maybe UltraEdit uses interally Column - Insert Column for adding a line comment and because removing with Column - Delete Column can't be done when 1 line in the selected block does not start with a line comment, UE analyzes line by line. Maybe I should suggest IDM to use internally always a regular expression replace for Comment Add/Remove.
The new GUI is currently slower than the previous one. The performance loss depends on your computer and the Windows configuration. On my 8 year old Pentium 4 with 2 GHz with an extremly optimized Windows XP for maximum performance the performance loss is not really noticeable, but some users already reported noticeable delays on their computers. I'm sure the IDM developers will look into it and make performance enhancements in future versions.
And please update to currently latest version 15.00.0.1046 of UltraEdit. There are several serious bugs fixed in currently latest version.