[modem]: VFS DTE is very slow (IDFGH-14045)

[david-cermak]

Answers checklist.

• I have read the documentation for esp-protocols components and the issue is not addressed there.
• I have updated my esp-protocols branch (master or release) to the latest version and checked that the issue is present there.
• I have searched the issue tracker for a similar issue and not found a similar issue.

General issue report

IDF version: espressif/esp-idf@457f651
ESP-MODEM version: c5653ff

Standard communication using UART DTE based on VFS works, but is extremely slow. It is not possible to perform TLS handshake with server side verification (not even with a locally hosted server).

STR:

• Test: https://github.com/espressif/esp-protocols/tree/master/components/esp_modem/test/target_ota
• Config: https://github.com/espressif/esp-protocols/blob/master/components/esp_modem/test/target_ota/sdkconfig.ci.3

in general, open any https page with DTE constructed with

esp-protocols/components/esp_modem/test/target_ota/main/ota_test.cpp

Lines 188 to 192 in c5653ff

	struct esp_modem_vfs_uart_creator uart_config = ESP_MODEM_VFS_DEFAULT_UART_CONFIG("/dev/uart/1");
	assert(vfs_create_uart(&uart_config, &dte_config.vfs_config) == true);
	auto dte = create_vfs_dte(&dte_config);
	esp_vfs_dev_uart_use_driver(uart_config.uart.port_num);