Thank you @CopperBot for your valuable feedback! You are right about the Netsocket libraries folders as well as in case of the mbed-os/platform/tests/* folder. Unfortunately, I am not allowed to edit (fix) the .mbedignore in my first post any more (I don’t know why because I changed it several time in the past). So below is the .mbedignore fixed according to your feedback to make the compilation even faster 
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.mbedignore:
/* Bootloader */
mbed-os/features/FEATURE_BOOTLOADER/*
/* BLE */
mbed-os/connectivity/drivers/ble/*
mbed-os/connectivity/FEATURE_BLE/*
/* Cellular */
mbed-os/connectivity/cellular/*
mbed-os/connectivity/drivers/cellular/*
mbed-os/connectivity/netsocket/source/Cellular*.*
/* Device Key */
mbed-os/drivers/device_key/*
/* Experimental /
mbed-os/platform/FEATURE_EXPERIMENTAL_API/*
/* FPGA */
mbed-os/features/frameworks/COMPONENT_FPGA_CI_TEST_SHIELD/*
/* Greentea client */
mbed-os/features/frameworks/greentea-client/*
/* LORAWAN */
mbed-os/connectivity/drivers/lora/*
mbed-os/connectivity/lorawan/*
/* LWIP */
mbed-os/connectivity/drivers/emac/*
mbed-os/connectivity/lwipstack/*
/* Mbed-client-cli */
mbed-os/features/frameworks/mbed-client-cli/*
/* MBED TLS */
mbed-os/connectivity/drivers/mbedtls/*
mbed-os/connectivity/mbedtls/*
/* Nanostack */
mbed-os/connectivity/drivers/emac/*
mbed-os/connectivity/libraries/mbed-coap/*
mbed-os/connectivity/libraries/nanostack-libservice/*
mbed-os/connectivity/libraries/ppp/*
mbed-os/connectivity/nanostack/*
/* Netsocket */
mbed-os/connectivity/drivers/emac/*
mbed-os/connectivity/netsocket/*
mbed-os/connectivity/libraries/mbed-coap/*
mbed-os/connectivity/libraries/ppp/*
/* NFC */
mbed-os/connectivity/drivers/nfc/*
mbed-os/connectivity/nfc/*
/* RF */
mbed-os/connectivity/drivers/802.15.4_RF/*
/* Storage */
mbed-os/storage/filesystem/*
mbed-os/storage/kvstore/*
mbed-os/storage/platform/*
/* Tests */
mbed-os/platform/tests/*
mbed-os/TEST_APPS/*
mbed-os/TESTS/*
mbed-os/UNITTESTS/*
/* Unity */
mbed-os/features/frameworks/unity/*
/* Utest */
mbed-os/features/frameworks/utest/*
/* USB */
mbed-os/drivers/usb/source/*
mbed-os/hal/usb/source/*
mbed-os/hal/usb/TARGET_Templates/*
/* WiFi */
mbed-os/connectivity/drivers/wifi/*