High CPU usage with BLEUART

There is high CPU usage observed when reading bytes from BLEUART connection. With modified (bigger) buffer size in `adafruit_ble/services/nordic.py`, if I read 180 bytes of BLE buffer in about 50 times per second, that uses the total CPU about 20%-25% with RaspberryPi4. RPi4 has 4 cores so that means it uses the single core about 80% to 100% percent.

Current configuration:
```
Raspberry Pi OS: 5.4.72-v7l+
Adafruit_CircuitPython_BLE: 7.3.1
Adafruit_Blinka_bleio: 2.0.4
Bluez: 5.50-1.2~deb10u1+rpt2
```

Start by reading the BLEUART implementation and the buffer handling in adafruit_ble/services/nordic.py. Reproduce the reported workload on a Raspberry Pi 4: read 180 bytes about 50 times per second with the larger buffer, then measure CPU usage. Done means identifying the source of the excessive CPU use and confirming lower usage under the same workload.

Written by the indexing model from the issue text.