SELECT 
  a.category_id 
FROM 
  cscart_categories as a 
  LEFT JOIN cscart_categories as b ON b.category_id IN (2916) 
WHERE 
  a.id_path LIKE CONCAT(b.id_path, '/%')

Query time 0.00412

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1485.80"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "b",
          "access_type": "const",
          "possible_keys": [
            "PRIMARY",
            "p_category_id"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "category_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.00",
            "eval_cost": "0.20",
            "prefix_cost": "0.00",
            "data_read_per_join": "5K"
          },
          "used_columns": [
            "category_id",
            "id_path"
          ]
        }
      },
      {
        "table": {
          "table_name": "a",
          "access_type": "index",
          "key": "id_path",
          "used_key_parts": [
            "id_path"
          ],
          "key_length": "767",
          "rows_examined_per_scan": 6944,
          "rows_produced_per_join": 771,
          "filtered": "11.11",
          "using_index": true,
          "cost_info": {
            "read_cost": "97.00",
            "eval_cost": "154.30",
            "prefix_cost": "1485.80",
            "data_read_per_join": "4M"
          },
          "used_columns": [
            "category_id",
            "id_path"
          ],
          "attached_condition": "(`webmarco`.`a`.`id_path` like <cache>(concat('2910/2916','/%')))"
        }
      }
    ]
  }
}

Result

category_id
2926
2931
2940
2951
2961
2967
2972
2978
2983
2989
2993
2998
3004
3010
3014
3017
3022
3028
3034
3040
3052
3064
3073
3079
3087
3092
3093
3099
3103
3096
3108
3118
3143
3146
3154
3160
3182
3189
3196
3204
3208
3215
3116
3140
3145
3149
3153
3159
3181
3187
3192
3197
3201
3206
3212
3217
3221
3234
3239
3244
3248
3237
3243
3250
3253
3256
3259
3264
3271
3275
3281
3270
3276
3282
3306
3308
3309
3312
3314
3317
3321
3326
3327
3331
3318
3323
3334
3335
3340
3344
3352
3357
3361
3379
3338
3343
3353
3359
3366
3384
3386
3401
3412
3414
3424
3433
3440
3447
3452
3456
3423
3425
3427
3431
3439
3445
3448
3453
3454
3459
3461
3462
3468
3469
3472
3478
3487
3488
3491
3498
3507
9595