When a 15-second time slot starts, I see other stations transmit for several seconds before my 6500 starts transmitting. Sometimes the rig goes into transmit, but I don't see any output for several seconds.
I didn't have this issue using JT65. Is this just how the FT8 protocol works, or perhaps it's a quirk between the 6500 and wsjt-x?
You can define a delay in milliseconds in the TX tab of the SETUP window.
Settings > Radio Setup ... > TX (tab)
This also allows you to set a sequence of the different RCA TX ports.
I have noticed the same thing when using PSDR. During a RTTY contest I notice that I get a delay between PTT and RF that gets longer and longer as time goes on. If I simply stop PSDR and restart it then the delay goes away. Same thing if I leave everything running without any transmissions for an hour or so.
I recently tried FT8 and I see the same behavior.
Flex 5000 + Win 10 (also W7) + Latest PSDR + N1MM+
Go to Task Manager, right click on the program you want to change priority on (SSDR, WSJT-X, DAX, ), then select "GO to Details" then select "Set Priority" and select "High". I've found SSDR and WSJT-X runs much smoother that way.