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 = 'en' 
WHERE 
  1 
  AND cscart_product_feature_variants.feature_id IN (18)

Query time 0.00047

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "78.80"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_feature_variants",
          "access_type": "ref",
          "possible_keys": [
            "feature_id"
          ],
          "key": "feature_id",
          "used_key_parts": [
            "feature_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 52,
          "rows_produced_per_join": 52,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "6.00",
            "eval_cost": "10.40",
            "prefix_cost": "16.40",
            "data_read_per_join": "59K"
          },
          "used_columns": [
            "variant_id",
            "feature_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "cscart_product_feature_variant_descriptions",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "satishecarter_dec_setup_001.cscart_product_feature_variants.variant_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 52,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "52.00",
            "eval_cost": "10.40",
            "prefix_cost": "78.80",
            "data_read_per_join": "157K"
          },
          "used_columns": [
            "variant_id",
            "lang_code"
          ]
        }
      }
    ]
  }
}