Mysql takes longer when sending data

I have this specific query that takes a lot of time to execute, other queries in the same tables run very fast. Querycache is included in mysql, but still the query below takes more than 80 seconds each time, and the processor crosses the 100% usage level.

I cannot change the request because it was created by Drupal. Is there anything else I can do to improve performance?

Inquiry :

select count(*) 
from (
    SELECT slk.key_id AS key_id 
    FROM slk slk  
        LEFT JOIN users users ON    slk.uid = users.uid 
        LEFT JOIN node node_users ON users.uid = node_users.uid 
            AND   node_users.type = 'profile'
) count_alias;

The following is the profile information :

+--------------------------------+-----------+
| Status                         | Duration  |
+--------------------------------+-----------+
| starting                       |  0.000029 | 
| checking query cache for query |  0.000093 | 
| Opening tables                 |  0.000210 | 
| System lock                    |  0.000007 | 
| Table lock                     |  0.000075 | 
| optimizing                     |  0.000008 | 
| statistics                     |  0.000113 | 
| preparing                      |  0.000027 | 
| executing                      |  0.000004 | 
| Sending data                   | 66.086903 | 
| init                           |  0.000027 | 
| optimizing                     |  0.000009 | 
| executing                      |  0.000018 | 
| end                            |  0.000003 | 
| query end                      |  0.000004 | 
| freeing items                  |  0.000049 | 
| storing result in query cache  |  0.000116 | 
| removing tmp table             |  0.033162 | 
| closing tables                 |  0.000106 | 
| logging slow query             |  0.000003 | 
| logging slow query             |  0.000085 | 
| cleaning up                    |  0.000007 | 
+--------------------------------+-----------+

explain in the request:

| id | select_type | table      | type   | possible_keys         | key     | key_len | ref             | rows  | Extra                        |
|  1 | PRIMARY     | NULL       | NULL   | NULL                  | NULL    | NULL    | NULL            |  NULL | Select tables optimized away | 
|  2 | DERIVED     | slk        | ALL    | NULL                  | NULL    | NULL    | NULL            | 55862 |                              | 
|  2 | DERIVED     | users      | eq_ref | PRIMARY               | PRIMARY | 4       | gscom.slk.uid   |     1 | Using index                  | 
|  2 | DERIVED     | node_users | ref    | node_type,uid,idx_ctp | uid     | 4       | gscom.users.uid |     3 |                              | 

idx_ctpis an index on ( uid, type).

The query cache works, and below is the statistics.

show variables like '%query_cache%';:

| Variable_name                | Value    |
| have_query_cache             | YES      | 
| query_cache_limit            | 2097152  | 
| query_cache_min_res_unit     | 4096     | 
| query_cache_size             | 52428800 | 
| query_cache_type             | ON       | 
| query_cache_wlock_invalidate | OFF      |

mysql> show status like '%Qcache%';:

| Variable_name           | Value    |
| Qcache_free_blocks      | 1255     | 
| Qcache_free_memory      | 22902848 | 
| Qcache_hits             | 1484908  | 
| Qcache_inserts          | 1036344  | 
| Qcache_lowmem_prunes    | 95086    | 
| Qcache_not_cached       | 3975     | 
| Qcache_queries_in_cache | 14271    | 
| Qcache_total_blocks     | 30117    |