Attached is a test case demonstrating the problem. If a big buffer has tags  
applied in many places (there are about 70 tags in total, and the buffer is  
9000 lines long, in this test case), then deleting range of text is incredibly  
slow - in attached test case it takes minutes.   
It's not really clear what 'big' and 'lot' mean. Attached test case is a dump  
of real text buffer with syntax highlighting inside. With some highlighting,  
deleting is not that slow; with some highlighting it is. I.e. just size of  
buffer and just number of places where tags are applied does not necessarily  
lead to slow deletion. But I could not track it down to something meaningful.  
Interesting thing is that removing tags from buffer before deleting makes  
things fast - it takes seconds. To see this, change '#if 0' to '#if 1' in the  
source.  
  
To reproduce:  
1) compile attached file;  
2) run it;  
3) press ctrl-a to select all, and press backspace.  
Result: it freezes for minutes.  
  
It reproduces in gtk-2.6.4 and gtk from gtk-2-8 cvs branch.