NSTextView lag during typing

Question

I have two NSTextViews with a shared custom NSTextStorage subclass. The textviews mirror each other contents. This works fine until I press enter. Then the second textview starts lagging or doesn't even show any new input. This happens regardless of which textview I type in.

How can I force the second textview to stay in sync with the first textview?

This is my NSTextStorage subclass.

@implementation MyTextStorage
{
    NSTextStorage *_backingStore;
}


-(id) init
{
    self = [super init];
    if (self)
    {
        _backingStore = [[NSTextStorage alloc] initWithString:@""];
    }
    return self;
}

-(NSString *) string
{
    return [_backingStore string];
}

-(NSDictionary *) attributesAtIndex:(NSUInteger) location
                     effectiveRange:(NSRangePointer) range
{
    NSMutableDictionary *attributes = [[_backingStore attributesAtIndex:location
                                                         effectiveRange:range] mutableCopy];

    if ([self displayMode] == MyTextStorageDisplayNormal)
    {
        [attributes setObject:[NSColor blackColor]
                       forKey:NSForegroundColorAttributeName];//remove syntax coloring
    }
    return attributes;
}


#pragma mark - editing
-(void) replaceCharactersInRange:(NSRange)range
                      withString:(NSString *)string
{
    [_backingStore replaceCharactersInRange:range
                                 withString:string];
    [self edited:NSTextStorageEditedCharacters
           range:range
  changeInLength:[string length] - range.length];
}


-(void) setAttributes:(NSDictionary *) attributes
                range:(NSRange) range
{
    [_backingStore setAttributes:attributes
                           range:range];

    [self edited:NSTextStorageEditedAttributes
           range:range
  changeInLength:0];
}