iRule adding 10% CPU overhead

when HTTP_REQUEST priority 1 {
  set static::maxActiveClients "[class search -all -value datagroup_selfridges_waitingroom starts_with "max_active_clients"]"
  set subtableName "sessionLimit"
  set sessionCookieName "[class search -all -value datagroup_selfridges_waitingroom starts_with "session_cookie_prefix"]"
  set holdingPage "[class search -all -value datagroup_selfridges_waitingroom starts_with "holding_page_url"]"
  set static::sessionTimeout "[class search -all -value datagroup_selfridges_waitingroom starts_with "session_timeout"]"

  set need_cookie 0
  if {[HTTP::cookie exists $sessionCookieName]} { 
    set client_id [HTTP::cookie $sessionCookieName]
    set sessiondata [table lookup -subtable $subtableName $client_id]
    if { $sessiondata != "" } {
      log local0. "Valid session $client_id - continuing"
      return
    }
  }

  log local0. "No session. Checking for free slot (Max $static::maxActiveClients)"
  set sessionCount [table keys -subtable $subtableName -count]

  log local0. "No session. Checking for free slot (Current $sessionCount)"
  if {$sessionCount < $static::maxActiveClients} {
    set need_cookie 1
    set client_id [format "%08d" [expr { int(1000000000 * rand()) }]]
    set sessionValue [IP::client_addr]

    table add -subtable $subtableName $client_id $sessionValue $static::sessionTimeout
    log local0. "New Session ($client_id) added value $sessionValue Timeout static::sessionTimeout"
  } else {
    HTTP::redirect $holdingPage
  }
}

when HTTP_RESPONSE priority 1 {
  if {$need_cookie == 1} {
    HTTP::cookie insert name $sessionCookieName value $client_id path "/"
  }
}