Windows 10 booting problem when I use softwareserial communication

Hello, I have some problem when I use my LP.

I am using software serial communication between UART port (for Intel CPU) and arduino leonardo.
Many of people say UART_RXD, UART_TXD is pin 5, pin 7 in UART port, so I am using that pins.
In arduino leonardo, because only several pin can be used for RX of software serial communication, I use arduino pin 8, pin 9 as RX, TX.
Between of them, logic level converter is used for voltage conversion (5V to 1.8V, 1.8V to 5V).
The booting problem occurs in here.

If I connect those ports after Windows booting, there is no problem. Serial communication also be operated well.
but, If those ports are connected before Windows booting, then Windows booting isn't proceed and stops. (Red led indicating booting status also was weak when those ports are connected before booting.)
It is possible to use LP by re-connecting those ports after every booting, but it is inconvenient.

Someone who knows about this problem, please notice to me the cause. thanks.