Seem to find a solution.

asyncio.create_task is made for running coroutine in the same event loop as the main process. And await create_task(CatalogHandler.catalog_check(user.group_name, req.source, req.csv)) blocks process because it makes event loop wait for function execution.

What I changed:

• Made CatalogHandler.catalog_check method sync.
• Changed calling it to await to_thread(CatalogHandler.catalog_check, user.group_name, req.source, req.csv). It makes function run in a separate thread without blocking main event loop.

And everything seems to work! Now I can execute a long-running process with websockets without blocking other API endpoints. Hope this is being useful. Will update this answer if I find anything interesting about the solution.