SELECT 
  SQL_CALC_FOUND_ROWS products.product_id, 
  IF(
    shared_descr.product_id IS NOT NULL, 
    shared_descr.product, descr1.product
  ) as product, 
  IF(
    shared_descr.product_id IS NOT NULL, 
    shared_descr.youtube_link, products.youtube_link
  ) AS youtube_link, 
  cscart_aval_statuses_descr.descr as aval_descr 
FROM 
  cscart_products as products 
  LEFT JOIN cscart_product_descriptions as descr1 ON descr1.product_id = products.product_id 
  AND descr1.lang_code = 'se' 
  LEFT JOIN cscart_product_prices as prices ON prices.product_id = products.product_id 
  AND prices.lower_limit = 1 
  INNER JOIN cscart_products_categories as products_categories ON products_categories.product_id = products.product_id 
  INNER JOIN cscart_categories ON cscart_categories.category_id = products_categories.category_id 
  AND (
    cscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_categories.usergroup_ids
    )
  ) 
  AND cscart_categories.status IN ('A', 'H') 
  LEFT JOIN cscart_ult_product_descriptions shared_descr ON shared_descr.product_id = products.product_id 
  AND shared_descr.company_id = 2 
  AND shared_descr.lang_code = 'se' 
  LEFT JOIN cscart_aval_statuses ON cscart_aval_statuses.aval_status_id = products.aval_status 
  LEFT JOIN cscart_aval_statuses_descr ON cscart_aval_statuses.aval_status_id = cscart_aval_statuses_descr.aval_status_id 
  AND cscart_aval_statuses.status = 'A' 
  AND cscart_aval_statuses_descr.lang_code = 'se' 
  LEFT JOIN cscart_product_popularity as popularity ON popularity.product_id = products.product_id 
WHERE 
  1 
  AND cscart_categories.category_id IN (951) 
  AND cscart_categories.company_id = 2 
  AND (
    products.usergroup_ids = '' 
    OR FIND_IN_SET(0, products.usergroup_ids) 
    OR FIND_IN_SET(1, products.usergroup_ids)
  ) 
  AND products.status IN ('A') 
  AND prices.usergroup_id IN (0, 0, 1) 
GROUP BY 
  products.product_id 
ORDER BY 
  popularity.total desc, 
  product_id ASC 
LIMIT 
  36, 12

Query time 0.00350

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_categories const PRIMARY,c_status,p_category_id,idx_category_id PRIMARY 3 const 1 Using temporary; Using filesort
1 SIMPLE products_categories ref PRIMARY,pt,product_id_idx,category_id_idx,product_category_idx,category_product_idx PRIMARY 3 const 53 Using where; Using index
1 SIMPLE products eq_ref PRIMARY,status,product_id_idx,status_idx PRIMARY 3 mahm3t_cs443.products_categories.product_id 1 Using where
1 SIMPLE prices ref usergroup,product_id,lower_limit,usergroup_id usergroup 3 mahm3t_cs443.products_categories.product_id 1 Using where; Using index
1 SIMPLE shared_descr eq_ref PRIMARY,product_id,company_id PRIMARY 13 mahm3t_cs443.products_categories.product_id,const,const 1 Using where
1 SIMPLE cscart_aval_statuses eq_ref PRIMARY PRIMARY 4 mahm3t_cs443.products.aval_status 1
1 SIMPLE cscart_aval_statuses_descr ref aval_status_id aval_status_id 10 mahm3t_cs443.cscart_aval_statuses.aval_status_id,const 1 Using where
1 SIMPLE popularity eq_ref PRIMARY,total PRIMARY 3 mahm3t_cs443.products_categories.product_id 1
1 SIMPLE descr1 eq_ref PRIMARY,product_id PRIMARY 9 mahm3t_cs443.products_categories.product_id,const 1 Using where

Result

product_id product youtube_link aval_descr
10210 Topphängt takfönster VELUX GPL 3068 | säkerhets 3-glas, klarlackat furu
9822 Pivåhängt takfönster VELUX GLU 0064 | 3-glas, Everfinish (träkärna med vitt hölje i polyuretan)
9821 Pivåhängt takfönster VELUX GLL 1064 | 3-glas, klarlackat furu <p>Skickas ut inom ung. 5-7 arbetsdagar efter mottagandet av din betalning</p>
8877 Topphängt takfönster VELUX GPL 3066 | säkerhets 3-glas, klarlackat furu
9823 Takfönster VELUX GLL-B 1064B | klarlackat furu med ljuddämpande 3-glas och bottenhandtag <p>Skickas ut inom ung. 5-7 arbetsdagar efter mottagandet av din betalning</p>
10206 Elektrisk takfönster VELUX GGL 3062 INTEGRA | 3-lglas, klarlackat furu
9824 Pivåhängt takfönster med nedre handtaget VELUX GLU-B 0064 | 3-glas, Everfinish (träkärna med vitt hölje i polyuretan)
9826 Takfönster ROTO DESIGNO R49K ✓ PVC ✓ med WD termo-block
10208 Elektrisk takfönster VELUX GGU 0062 INTEGRA | 3-glas, Everfinish (träkärna med vitt hölje i polyuretan)
9825 Takfönster ROTO DESIGNO R49H ✓ trä ✓ med WD termoblock
10165 Takfönster FAKRO FTT-U R3
1646 Takfönster FAKRO FTT R3 | extra ljudisolering