How to Improve Query Performance with many JOINs

I have a query (with the purpose of making a view) which is using a few joins to get each column. Performance degrades quickly (exponentially?) for each set of joins added.

What would be a good approach to make this query faster? Please see comments within the query.

If it helps, this is using the WordPress DB schema.

Here is a screenshot of EXPLAIN

PRODUCTS TABLE

+--+----+
|id|name|
+--+----+
|1 |test|
+--+----+

METADATA TABLE

+----------+--------+-----+
|product_id|meta_key|value|
+----------+--------+-----+
|1         |price   |9.99 |
+----------+--------+-----+
|1         |sku     |ABC  |
+----------+--------+-----+

TERM_RELATIONSHIPS TABLE

+---------+----------------+
|object_id|term_taxonomy_id|
+---------+----------------+
|1        |1               |
+---------+----------------+
|1        |2               |
+---------+----------------+

TERM_TAXONOMY TABLE

+----------------+-------+--------+
|term_taxonomy_id|term_id|taxonomy|
+----------------+-------+--------+
|1               |1      |size    |
+----------------+-------+--------+
|2               |2      |stock   |
+----------------+-------+--------+

TERMS TABLE

+-------+-----+
|term_id|name |
+-------+-----+
|1      |500mg|
+-------+-----+
|2      |10   |
+-------+-----+

QUERY

SELECT 
  products.id,
  products.name,
  price.value AS price,
  sku.value AS sku,
  size.name AS size
FROM products

/* These joins are performing quickly */

INNER JOIN `metadata` AS price ON products.id = price.product_id AND price.meta_key = 'price'
INNER JOIN `metadata` AS sku ON products.id = sku.product_id AND sku.meta_key = 'sku'

/* Here's the part that is really slowing it down - I run this chunk about 5 times with different strings to match */

INNER JOIN `term_relationships` AS tr ON products.id = tr.object_id
  INNER JOIN `term_taxonomy` AS tt
  ON tr.term_taxonomy_id = tt.term_taxonomy_id AND tt.taxonomy = 'size'
    INNER JOIN `terms` AS size
    ON tt.term_id = size.term_id