When measuring the speed using iperf, the speed does not exceed 30 Mbps

How about this:

• Upgrade to a higher-performance VM SKU, such as D-series, E-series, or F-series, which offer better network throughput.
• Enable Accelerated Networking if supported by your VM size.
• Test within the same region and VNET to avoid internet routing limitations.
• Use multiple parallel iperf streams to fully utilize available bandwidth.