Testing with a wallet with a ~1000 unpaid/expired payment requests,

with the Qt gui, the python process saturates one of my CPU cores to 100%.
This is because main_window.timer_actions() calls request_list.refresh_all()
every 0.5 seconds, which iterates over all unpaid payreqs, and does a
scriptpubkey->address->scriptpubkey conversion.

Trace of the hot path:
```
  File "/home/user/wspace/electrum/./run_electrum", line 510, in <module>
    main()
  File "/home/user/wspace/electrum/./run_electrum", line 421, in main
    handle_cmd(
  File "/home/user/wspace/electrum/./run_electrum", line 439, in handle_cmd
    d.run_gui(config, plugins)
  File "/home/user/wspace/electrum/electrum/daemon.py", line 581, in run_gui
    self.gui_object.main()
  File "/home/user/wspace/electrum/electrum/gui/qt/__init__.py", line 469, in main
    self.app.exec_()
  File "/home/user/wspace/electrum/electrum/gui/qt/main_window.py", line 861, in timer_actions
    self.receive_tab.request_list.refresh_all()
  File "/home/user/wspace/electrum/electrum/gui/qt/util.py", line 821, in refresh_all
    self.refresh_row(key, row)
  File "/home/user/wspace/electrum/electrum/gui/qt/request_list.py", line 126, in refresh_row
    status = self.wallet.get_invoice_status(request)
  File "/home/user/wspace/electrum/electrum/wallet.py", line 2330, in get_invoice_status
    paid, conf = self.is_onchain_invoice_paid(invoice)
  File "/home/user/wspace/electrum/electrum/wallet.py", line 1118, in is_onchain_invoice_paid
    is_paid, conf_needed, relevant_txs = self._is_onchain_invoice_paid(invoice)
  File "/home/user/wspace/electrum/electrum/wallet.py", line 1085, in _is_onchain_invoice_paid
    outputs = invoice.get_outputs()
  File "/home/user/wspace/electrum/electrum/invoices.py", line 130, in get_outputs
    traceback.print_stack()
```

These commits drastically reduce the CPU utilisation.

electrum/gui/qt/util.py

@@ -813,6 +813,8 @@ class MyTreeView(QTreeView):
                 return row
 
     def refresh_all(self):
+        if self.maybe_defer_update():
+            return
         for row in range(0, self.std_model.rowCount()):
             item = self.std_model.item(row, 0)
             key = item.data(self.key_role)

electrum/invoices.py

@@ -1,5 +1,5 @@
 import time
-from typing import TYPE_CHECKING, List, Optional, Union, Dict, Any
+from typing import TYPE_CHECKING, List, Optional, Union, Dict, Any, Sequence
 from decimal import Decimal
 
 import attr
@@ -126,16 +126,13 @@ class Invoice(StoredObject):
             address = self._lnaddr.get_fallback_address() or None
         return address
 
-    def get_outputs(self):
-        if self.is_lightning():
+    def get_outputs(self) -> Sequence[PartialTxOutput]:
+        outputs = self.outputs or []
+        if not outputs:
             address = self.get_address()
             amount = self.get_amount_sat()
             if address and amount is not None:
                 outputs = [PartialTxOutput.from_address_and_value(address, int(amount))]
-            else:
-                outputs = []
-        else:
-            outputs = self.outputs
         return outputs
 
     def can_be_paid_onchain(self) -> bool:

electrum/transaction.py

@@ -148,9 +148,18 @@ class TxOutput:
             return cls(scriptpubkey=bfh(addr), value=val)
         raise Exception(f"unexpected legacy address type: {_type}")
 
+    @property
+    def scriptpubkey(self) -> bytes:
+        return self._scriptpubkey
+
+    @scriptpubkey.setter
+    def scriptpubkey(self, scriptpubkey: bytes):
+        self._scriptpubkey = scriptpubkey
+        self._address = get_address_from_output_script(scriptpubkey)
+
     @property
     def address(self) -> Optional[str]:
-        return get_address_from_output_script(self.scriptpubkey)  # TODO cache this?
+        return self._address
 
     def get_ui_address_str(self) -> str:
         addr = self.address