openOCD : how do I know if my LPT-JTAG adapter is in sync?
Does this confirms if the JTAG-adapter is in sync with target?


OpenOCD would otherwise print diagnostic messages: Polling target failed [...].


  How/Where to reduce the adapter and target communication speed


It is already at a low 10 kHz. You can change it in ther terminal (port 4444) or in the config file with:

adapter_khz 500


More than 500 kHz is not recommended - the MCU runs at 4 MHz by default and requires JTAG clock to be lower than CCLK/4.