Why does setFormat in hightlight take 20 times longer when opening a document than when inserting text?

Question

Syntax highlighting

Given:
Yes . It is set in __init__ among other thingsclass MyEdit(QPlainTextEdit)

self.highlight = HL_Syntax.GMHighlighter(self.document())

Is , for which it is overriddenclass GMHighlighter (QSyntaxHighlighter)

def highlightBlock(self, text):
...
self.setFormat(index, length, format)

And there is also a document for syntax highlighting for a million lines.
Now, actually, an unclear moment:
Without

self.highlight = HL_Syntax.GMHighlighter(self.document())

it takes 6 seconds to cleanly open and place MyEdit a millionaire file , it takes 14 seconds to insert the same text into an empty one . That is, everything is relatively fast, but opening is faster than Ctrl + V . If you turn on syntax recognition, but do not turn on visual highlighting (without ), then 35 seconds are added to both results , which is logical. However, it is worth turning it on and strangeness begins - Ctrl + V adds another six seconds , and opening the file instantly slows down at times and takes more than three minutes .MyEdit
setFormat
self.setFormat(index, length, format)
That is, creating a new file and pasting text into it becomes faster than opening a text file, although opening itself took less time than pasting.
Found that the problem exists when using self.updateRequest.connect(self.updateLineNumberArea) and seFormat at the same time, where

def updateLineNumberArea(self, rect, dy):
        if dy:
            self.lineNumberArea.scroll(0, dy)
        else:
            self.lineNumberArea.update(0, rect.y(), self.lineNumberArea.width(), rect.height())
        if rect.contains(self.viewport().rect()):
            self.updateLineNumberAreaWidth(0)

Why is this happening and what can help me?