xlsx which was opened before in less than 1 second now opens about 15-20 mins
*Created originally on Bitbucket by [zerg43 (Anirain)](https://bitbucket.org/%7B1baadaeb-e866-4d86-b82d-7c9059a155e9%7D/)*
In attached file calls of merge_cells at worksheet/worksheet.py from parse_merge() for cycle from reader/worksheet.py causing slow down, especially at the end of parse_merge cycle. In this case it has 13515 merged.mergeCell, near the last iterations one cycle lasts about ~0.5 seconds. File had data range in 63 cols, 463 rows, I deleted it, cause it does not matter for this case.
Uploading also worksheet.py with prints just to see it.
Tried on:
windows 10/64 bit/ python 3.6.4/lxml 4.2.1.0/openpyxl 2.5.3
ubuntu 16.04/64 bit/ python 3.6.3/last lxml/last openpyxl
Using this code to open xlsx:
```
#!python
# -*- coding: utf-8 -*-
from openpyxl import load_workbook
file_name = r'd:\thiska.xlsx'
wb = load_workbook(file_name) # <- this taking too long time
ws = wb.active
```
*Attachments:* [thiska.xlsx](/uploads/4fd8f95dc04d85b73473e89c74d0fca3/thiska.xlsx)[worksheet.py](/uploads/5cc8f8174c2dbdb9a7d4ac905bc84bbe/worksheet.py)
issue