optimize slice allocation in shutter crypto #6799

Job	Run time
test-hive (rpc, compat, 23)	3m 0s
test-hive (engine, auth, 0)	3m 43s
test-hive (engine, api, 0)	7m 6s
test-hive (engine, withdrawals, 0)	6m 42s
test-hive (engine, cancun, 3)	8m 58s
test-hive (engine, exchange-capabilities, 0)	3m 47s
	33m 16s

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