WJST 2.4.0 Delay issue (split/rig/fake it) fix until next release
As many know at this point, there was / is a delay on PTT for the K3/S related to the version of HamLib that ships with WSJT 2.4.0. The solution is to set the rig split mode to none which then defeats the advantage of transmitting a cleaner signal (tx harmonics are suppressed by the vfo b's sideband filter) The problem exists for 'rig' or 'fake it' mode in my testing.
I resolved this issue by updating the HamLib dll's in the 2.4.0 release of WSJT. A new feature / change to WSJT 2.4.0 was that for Windows they use the dll interface to HamLib. (see the release notes) This allows you to update the version of HamLib that ships with WSJT. So if you like to use the rig split / fake it option in WSJT with your K3/S and are capable of backing up your WSJT ../wsjt/bin directory and downloading a zip file and copying some files from the zip file to your ../wsjt/bin folder you can fix the delay on PTT issue before the next release / beta release of WSJT where they ship a newer version of HamLib that will fix the problem with the K3/S.
As in all things computers, stupid is as stupid does. Back up your current WSJT install / or at least the DLL's you will replace or don't do this... i.e. just continue to use 'none' for your rig split option in WSJT and wait for the next release of WSJT or beta / RC.
So if you are still reading, then you will need to download the daily build of HamLib for your Windows platform, 32 bit or 64 bit here: N0NB SF.net developer page (sourceforge.net)
Extract the zip file to a temp location on your computer etc... and in the ../bin folder copy the 4 dll's to your ../wsjt/bin folder (after shutting down WSJT of course and backing up your ../wsjt/bin folder). Restart WSJT and the problem with the delay was solved for me. I'm back using the rig split option (in my case I'm using a com port for PTT to my K3S for what it's worth).
Anyway, it's nice with the latest release that we now have the option of updating HamLib post WSJT release with the use of the dll interface to HamLib. This is a perfect example of HamLib introducing an issue, but a later daily build of HamLib had the issue fixed. I'm using the 06-15 daily release build of HamLib and I copied these zip files and hosted them here if you want to stick with the daily build I tested: www.nc7j.com - /downloads/NG7M/HamLib/
You mileage may vary... it's working great for me with the daily 6-15-2021 build of HamLib. If FUD is your mantra in Ham Radio, then don't use this work around, wait for the WSJT release that will have the newer HamLib version.
See section 4.2 of the WSJT User guide if you want to read about the split and fake it option which can give you a cleaner transmitted signal: WSJT-X 2.0 User Guide (princeton.edu)