diff --git a/electrum/bitcoin.py b/electrum/bitcoin.py
index 7afde6e54c7..b9906ce72f5 100644
--- a/electrum/bitcoin.py
+++ b/electrum/bitcoin.py
@@ -539,7 +539,10 @@ class BaseDecodeError(BitcoinException): pass
 
 
 def base_encode(v: bytes, *, base: int) -> str:
-    """ encode v, which is a string of bytes, to base58."""
+    """ encode v, which is a string of bytes, to base58.
+
+    note: time complexity is O(len(v)^2), due to big-int arithmetic.
+    """
     assert_bytes(v)
     if base not in (58, 43):
         raise ValueError('not supported base: {}'.format(base))
@@ -552,10 +555,11 @@ def base_encode(v: bytes, *, base: int) -> str:
     newlen = len(v)
 
     num = int.from_bytes(v, byteorder='big')
-    string = b""
+    string_rev = bytearray()
     while num:
         num, idx = divmod(num, base)
-        string = chars[idx:idx + 1] + string
+        string_rev += chars[idx:idx + 1]
+    string = string_rev[::-1]
 
     result = chars[0:1] * (origlen - newlen) + string
     return result.decode('ascii')
@@ -564,6 +568,8 @@ def base_encode(v: bytes, *, base: int) -> str:
 def base_decode(v: Union[bytes, str], *, base: int) -> Optional[bytes]:
     """ decode v into a string of len bytes.
 
+    note: time complexity is O(len(v)^2), due to big-int arithmetic.
+
     based on the work of David Keijser in https://github.com/keis/base58
     """
     # assert_bytes(v)
diff --git a/electrum/transaction.py b/electrum/transaction.py
index 46b9351c765..3641e639935 100644
--- a/electrum/transaction.py
+++ b/electrum/transaction.py
@@ -1228,7 +1228,8 @@ def to_qr_data(self) -> Tuple[str, bool]:
             tx.convert_all_utxos_to_witness_utxos()
             is_complete = False
         tx_bytes = tx.serialize_as_bytes()
-        return base_encode(tx_bytes, base=43), is_complete
+        tx_base43 = base_encode(tx_bytes, base=43)  # FIXME this takes quadratic time in len(tx)
+        return tx_base43, is_complete
 
     def txid(self) -> Optional[str]:
         if self._cached_txid is None:
@@ -1499,17 +1500,26 @@ def convert_raw_tx_to_hex(raw: Union[str, bytes]) -> str:
         return binascii.unhexlify(raw).hex()
     except Exception:
         pass
-    # try base43
-    try:
-        return base_decode(raw, base=43).hex()
-    except Exception:
-        pass
     # try base64
     if raw[0:6] in ('cHNidP', b'cHNidP'):  # base64 psbt
         try:
             return base64.b64decode(raw, validate=True).hex()
         except Exception:
             pass
+    # try base43
+    try:
+        # FIXME This takes quadratic time in len(tx).
+        #       We could prefix all txs we base43-serialize with e.g. "BASE43TX:",
+        #       (and break-compat with old versions).  Then at least we would not attempt
+        #       the expensive deser here if it's not needed.
+        if len(raw) > 30_000:
+            # note: base_decode for this length takes around 0.2 sec on my laptop.
+            # note: We only use/expect base43 inside QR codes. The max data a QR can fit is around 4 KB,
+            #       serializing that to b43 results in a length of ~5500. 30k is already over 5x that.
+            raise ValueError("raw tx too large for base43")
+        return base_decode(raw, base=43).hex()
+    except Exception:
+        pass
     # raw bytes
     if isinstance(raw, (bytes, bytearray)):
         return raw.hex()