Performance problem while making a rich text editor in xamarin - performance

Question

We want to make a rich (but not too rich) text editor with xamarin such that it is possible to:
Highlight some words
Position the cursor anywhere with the mouse or touch
Show the position of the cursor
Our attempt is using a Label to display text, and when the user click on the label in reality a hidden Entry is focused. To reposition the cursor we split the FormattedString in more Span, one for each character, so we know which is tapped or clicked.
The problem now is performance, as the content grows the time of updating the view grows a lot. For example just for adding one character can take 3000 ms if the content length is around 30 character. By the way it takes just 10 ms if the content length is little like 1-5 characters. The time to update the label is not linear on the length of text.
Here I put the code where the performance problem occur, and I want ask you if there is a problem in my code or maybe I can't do this in xamarin in this way.
Stopwatch stopwatch = Stopwatch.StartNew();
string newText = e.NewTextValue;
string oldText = (e.OldTextValue ?? string.Empty);
List<string> newParts = newText.Split('#').ToList();
List<string> oldParts = oldText.Split('#').ToList();
// ...
foreach (string part in newParts)
{
string placeholderCode = $"#{part}#";
string textToShow = isPlaceholder ? placeholders[placeholderCode] : part;
if (isPlaceholder)
{
editorPosition += placeholderCode.Length;
}
Span tmpSpan = new Span();
foreach (char character in textToShow)
{
Span item = viewer.FormattedText.Spans.ElementAtOrDefault(currentSpan++);
if (item == null)
{
item = new Span();
TapGestureRecognizer gestureRecognizer = new TapGestureRecognizer();
gestureRecognizer.Tapped += delegate {
editor.CursorPosition = spanToEditorPosition[item];
FocusEditor();
};
item.GestureRecognizers.Add(gestureRecognizer);
viewer.FormattedText.Spans.Add(item);
}
if (isPlaceholder)
{
tmpSpan.FontAttributes = FontAttributes.Bold;
}
else
{
tmpSpan.FontAttributes = FontAttributes.None;
editorPosition++;
}
tmpSpan.Text = character.ToString();
if (tmpSpan.Text != item.Text || tmpSpan.FontAttributes != item.FontAttributes)
{
item.Text = tmpSpan.Text;
item.FontAttributes = tmpSpan.FontAttributes;
}
spanToEditorPosition.Add(item, editorPosition);
}
isPlaceholder = !isPlaceholder;
}
stopwatch.Stop();