Please test for-4.19/block:
https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-block.git/log/?h=for-4.19/block
This slow boot issue should have been fixed by the following commits:
1311326cf475 blk-mq: avoid to synchronize rcu inside blk_cleanup_queue()
97889f9ac24f blk-mq: remove synchronize_rcu() from blk_mq_del_queue_tag_set()
5815839b3ca1 blk-mq: introduce new lock for protecting hctx->dispatch_wait
2278d69f030f blk-mq: don't pass **hctx to blk_mq_mark_tag_wait()
8ab6bb9ee8d0 blk-mq: cleanup blk_mq_get_driver_tag()