SELECT 
  company_id, 
  storefront 
FROM 
  cscart_companies 
WHERE 
  FIND_IN_SET('US', countries_list) 
LIMIT 
  1

Query time 0.00014

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_companies",
      "access_type": "ALL",
      "rows": 1,
      "filtered": 100,
      "attached_condition": "find_in_set('US',cscart_companies.countries_list)"
    }
  }
}