is_ignored_block_opening(config, item) and is_safe_closing_tag(config, item) are each called multiple times in indent.py with the same parameters. This can be deduplicated.

(Note I've changed the block with executor_cls(max_workers=worker_count) as exe: in __init__.py to be serial to make the profiler work.

for this_file in file_list:
  file_errors.append(process(config, this_file))
  # if temp_file block etc. is removed

Before:

is_ignored_block_opening (With children): 12.4% of all the time, 61562x calls
is_safe_closing_tag (With children): 6.24% of all the time, 33139x calls

Concrete data:

Netbox with serial execution (Patch above): 9.7s
Netbox with unchanged __init__.py              :    4s
OpenEDX platform                                           : 33s

After

is_ignored_block_opening (With children): 4.73% of all the time, 20622x calls
is_safe_closing_tag (With children): 4.61% of all the time, 20941x calls

Concrete data:

Netbox with serial execution (Patch above): 8.3s
Netbox with unchanged __init__.py              : 3.5s
OpenEDX platform                                           : 26s