Yes the 5V reference voltage on the Nano-10 can source 10mA of current to your temperature sensor. Since it only takes 50uA there is no problem at all.
Is it very critical to be able to see the "seconds" being updated on the HMI every second?
You can try to connect to the PLC via the Ethernet if your HMI support it. Ethernet communication is significantly faster than serial communication.
If you must use serial comm, you can try to increase the baud rate of both the PLC and the HMI to see if that makes a difference.