Regression in copy records performance between 0.30.0 and 0.31.0

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When run on different versions (0.30.0 vs 0.31.0), the following script reproduces the issue and consistently shows a performance regression of approximately 22%:

```py
import asyncio
import asyncpg
import time

COUNT = 5_000

async def run():
conn = await asyncpg.connect("postgresql://user:password@localhost/postgres")

await conn.execute("CREATE TABLE IF NOT EXISTS users (id SERIAL PRIMARY KEY, username TEXT, email TEXT, age INT)")

RECORDS = [("John Doe", "john.doe@example.com", 42)] * 10_000

start = time.perf_counter()
for _ in range(COUNT):
await conn.copy_records_to_table("users", records=RECORDS, columns=["username", "email", "age"])
end = time.perf_counter()

await conn.close()

return end - start

if __name__ == "__main__":
total = asyncio.run(run())
print(f"Total time: {total:.5f} seconds")
print(f"Average time per loop: {total / COUNT:.8f} seconds")
```

```
(common) [e.lee@OC0000842]$ pip install asyncpg==0.30.0
(common) [e.lee@OC0000842]$ python -m example
Total time: 89.15468 seconds
Average time per loop: 0.01783094 seconds

(common) [e.lee@OC0000842]$ pip install asyncpg==0.31.0
(common) [e.lee@OC0000842]$ python -m example
Total time: 108.57080 seconds
Average time per loop: 0.02171416 seconds
```

Start by running the supplied asyncio script with asyncpg 0.30.0 and 0.31.0 against PostgreSQL, confirming the reported copy_records_to_table timing difference. Trace the copy_records_to_table path to identify what changed between those versions; done means the regression is explained and the benchmark no longer shows the reported slowdown.

Written by the indexing model from the issue text.