Original backported commit message:
> Re total runtime of WalletDB.write() and file size on disk,
> for a large encrypted wallet, compare:
>
> before (zlib level=6):
> file size 16_670 KB
> JsonDB.dump 0.5099 sec
> zlib.compress 1.3280 sec
> ECPubkey.encrypt_message 0.1720 sec
>
> after change (zlib level=1):
> file size 17_527 KB
> JsonDB.dump 0.5344 sec
> zlib.compress 0.5320 sec
> ECPubkey.encrypt_message 0.1837 sec
My own test confirms the gain in speed when writing an encrypted wallet file to disk:
before
file size: 959K
```
pierre@simak:~/dev/bitcoin-abc/electrum$ ./electrum-abc -v
| 7.516| |04| [profiler] WalletStorage.write 0.0644
| 16.291| |00| [profiler] WalletStorage.write 0.0682
```
after
filesize: 1018K (6% increase)
```
pierre@simak:~/dev/bitcoin-abc/electrum$ ./electrum-abc -v
| 9.092| |04| [profiler] WalletStorage.write 0.0343
| 17.637| |00| [profiler] WalletStorage.write 0.0399
```
Depends on D14234
This is a backport of https://github.com/spesmilo/electrum/commit/d86138a1a5a144b5adc32ef8bd1ff7afd860927d