Email header with ;;;; stuffing takes very long to parse

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-01-12.13:13:10.953>
labels = ['3.8', 'expert-email', '3.10', 'performance', '3.7', '3.9']
title = 'Email header with ;;;; stuffing takes very long to parse'
updated_at = <Date 2021-07-09.22:18:24.963>
user = 'https://github.com/eriker-fsc'

bugs.python.org fields:

activity = <Date 2021-07-09.22:18:24.963>
actor = 'andrei.avk'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['email']
creation = <Date 2021-01-12.13:13:10.953>
creator = 'eriker'
dependencies = []
files = ['49736']
hgrepos = []
issue_num = 42909
keywords = []
message_count = 3.0
messages = ['384931', '384938', '397228']
nosy_count = 4.0
nosy_names = ['barry', 'r.david.murray', 'andrei.avk', 'eriker']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'resource usage'
url = 'https://bugs.python.org/issue42909'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']