SELECT 
  COUNT(*) 
FROM 
  cscart_product_feature_variants 
  LEFT JOIN cscart_product_feature_variant_descriptions ON cscart_product_feature_variant_descriptions.variant_id = cscart_product_feature_variants.variant_id 
  AND cscart_product_feature_variant_descriptions.lang_code = 'ru' 
WHERE 
  1 
  AND cscart_product_feature_variants.feature_id IN (18)

Query time 0.00066

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_feature_variants",
          "access_type": "ALL",
          "possible_keys": ["feature_id"],
          "rows": 158,
          "filtered": 33.54430389,
          "attached_condition": "cscart_product_feature_variants.feature_id = 18"
        }
      }
    ]
  }
}