Import Tablespace takes too long on a busy server

Even though with small tables, import tablespace command takes too long to complete in a busy slave server with slave_parallel_workers > 1. pt-pmp output shows always same following function calls for import tablespace thread:

2 nanosleep,os_thread_sleep(os0thread.cc:303),buf_flush_wait_flushed(buf0flu.cc:2155),log_preflush_pool_modified_pages(log0log.cc:1581),log_make_checkpoint_at(log0log.cc:1581),row_import_cleanup(row0import.cc:2188),row_import_for_mysql(row0import.cc:4005),ha_innobase::discard_or_import_tablespace(ha_innodb.cc:13150),mysql_discard_or_import_tablespace(sql_table.cc:6316),Sql_cmd_discard_import_tablespace::execute(sql_alter.cc:391),mysql_execute_command(sql_parse.cc:5155),mysql_parse(sql_parse.cc:5929),dispatch_command(sql_parse.cc:1493),do_command(sql_parse.cc:1021),handle_connection(connection_handler_per_thread.cc:312),pfs_spawn_thread(pfs.cc:2188),start_thread,clone

It seems the thread is hung waiting for sync flushing. buffer_flush_sync_waits metric starts incrementing till the command finishes, as follows:

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Even with a busy server the import tablespace command shouldn't take so long to complete. 

MySQL Community 5.7.20 can also take long to complete, but it's not showing same function calls and also buffer_flush_sync_waits is not incremented.

2 pthread_cond_wait,wait(os0event.cc:165),os_event::wait_low(os0event.cc:165),buf_dblwr_add_to_batch(buf0dblwr.cc:1110),buf_flush_write_block_low(buf0flu.cc:1099),buf_flush_page(buf0flu.cc:1099),buf_flush_try_neighbors(buf0flu.cc:1453),buf_flush_page_and_try_neighbors(buf0flu.cc:1453),buf_do_flush_list_batch(buf0flu.cc:1780),buf_flush_batch(buf0flu.cc:1780),buf_flush_do_batch(buf0flu.cc:1780),buf_flush_lists(buf0flu.cc:2124),log_preflush_pool_modified_pages(log0log.cc:1510),log_make_checkpoint_at(log0log.cc:1902),row_import_cleanup(row0import.cc:2185),row_import_for_mysql(row0import.cc:4002),ha_innobase::discard_or_import_tablespace(ha_innodb.cc:12311),mysql_discard_or_import_tablespace(sql_table.cc:6050),Sql_cmd_discard_import_tablespace::execute(sql_alter.cc:369),mysql_execute_command(sql_parse.cc:4894),mysql_parse(sql_parse.cc:5612),dispatch_command(sql_parse.cc:1461),do_command(sql_parse.cc:999),handle_connection(connection_handler_per_thread.cc:300),pfs_spawn_thread(pfs.cc:2188),start_thread,clone

Please find attached file to repeat this case.

Regards.