SELECT 
  pfv.feature_id, 
  pfv.product_id, 
  pfv.variant_id, 
  gp.group_id 
FROM 
  cscart_product_features_values AS pfv 
  INNER JOIN cscart_product_variation_group_products AS gp ON pfv.product_id = gp.product_id 
  INNER JOIN cscart_product_variation_group_features AS gpf ON gpf.group_id = gp.group_id 
  AND gpf.feature_id = pfv.feature_id 
WHERE 
  pfv.lang_code = 'ru' 
  AND gp.group_id IN (835)

Query time 0.00984

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "gpf",
          "access_type": "ref",
          "possible_keys": ["PRIMARY", "idx_group_id"],
          "key": "idx_group_id",
          "key_length": "3",
          "used_key_parts": ["group_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "gp",
          "access_type": "ref",
          "possible_keys": ["PRIMARY", "idx_group_id"],
          "key": "idx_group_id",
          "key_length": "3",
          "used_key_parts": ["group_id"],
          "ref": ["const"],
          "rows": 89,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "pfv",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "idx_product_feature_variant_id",
          "key_length": "12",
          "used_key_parts": ["product_id", "feature_id", "lang_code"],
          "ref": [
            "lillyshop_lillyshop.gp.product_id",
            "lillyshop_lillyshop.gpf.feature_id",
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "pfv.lang_code = 'ru'",
          "using_index": true
        }
      }
    ]
  }
}

Result

feature_id product_id variant_id group_id
84 19925 3089 835
84 19883 3686 835
84 20087 4981 835
84 19888 5029 835
84 19879 3093 835
84 19878 3084 835
84 19905 3085 835
84 19820 3104 835
84 19860 3104 835
84 19863 3104 835
84 19865 3104 835
84 19866 3104 835
84 19867 3098 835
84 19868 3096 835
84 19869 3096 835
84 19870 3089 835
84 19871 3089 835
84 19872 3089 835
84 19873 3085 835
84 19874 3085 835
84 19875 3089 835
84 19876 3096 835
84 19877 3092 835
84 19880 3099 835
84 19881 3099 835
84 19882 3087 835
84 19884 5063 835
84 19885 3097 835
84 19886 3100 835
84 19887 3100 835
84 19889 5029 835
84 19890 3089 835
84 19891 3089 835
84 19892 4974 835
84 19893 3096 835
84 19894 3096 835
84 19895 3096 835
84 19896 4915 835
84 19897 4915 835
84 19898 4915 835
84 19899 3089 835
84 19900 3089 835
84 19901 4915 835
84 19902 3089 835
84 19903 3089 835
84 19904 3089 835
84 19906 3089 835
84 19907 3096 835
84 19908 3085 835
84 19909 3095 835
84 19910 3092 835
84 19911 3085 835
84 20154 3104 835
84 20723 3089 835
84 20724 5029 835
84 20725 3100 835
84 20726 3099 835
84 20727 3090 835
84 20728 3095 835
84 20065 3089 835
84 21078 4983 835
84 18661 3085 835
84 18662 3099 835
84 20088 3092 835
84 20089 3095 835
84 20091 3089 835
84 20092 3102 835
84 20093 3096 835
84 20094 3099 835
84 20971 3094 835
84 20972 3100 835
84 20973 3093 835
84 20974 3097 835
84 21923 3085 835
84 21924 3085 835
84 21102 3087 835
84 21103 3101 835
84 21749 3089 835
84 18664 3089 835
84 19131 3096 835
84 20576 3092 835
84 20955 3093 835
84 20956 3100 835
84 20957 4983 835
84 20958 4983 835
84 20959 3087 835
84 21925 3085 835
84 21926 3084 835
84 22277 3104 835