After some other users pointed to the slow response of test window when there is an error in the script, I installed back PSD 11 version and compared its performance to the newest PSD 12. And the difference is big - it seems that there is some overhead time cost to every communication/operation with the DB. Even the simple 'Select 1 from dual' takes average 0.368s in PSD12 and only 0.017s in PSD11. The error detection in test window takes 12s in PSD12 and 2s in PSD11.

Is there anything I can do to help you find out what's causing this?

I am using latest Win10 64bit, Oracle 11.2.

There is no difference in query processing performance between version 11 and 12 when I try it.

Can you go to Help > Support Info and send me the text from the "Info" tab page? Can you do this both for version 11.0 and 12.0?

Information sent to 'support@allroundautomations.com'
I see no major difference between ver. 11 and 12.

This looks okay. To obtain some more diagnostic information, can you modify the PL/SQL Developer shortcut and add the DebugSQL parameter? For example:

"C:\Program Files\PLSQL Developer\plsqldev.exe" DebugSQL

Reproduce the problem and send me the debug.txt file that is generated %APPDATA%\PLSQL Developer directory (e.g. C:\Users\<User>\AppData\Roaming\PLSQL Developer).

For version 12 these directories have a "12" suffix.

Based on the content of debug.txt files, I saw that some slowness maybe due to my too big logon history or other configuration problems. I deleted the configuration (user.prefs) and imported the saved connections again and now the PSD12 startup seems faster.
Even the 'select 1 from dual' is now faster (0.114 s) - still slower than PSD11 but maybe PSD12 is computing the time in a different way or the new GUI needs a little bit more time to show the results?

When I try some benchmark like this:

declare
i integer;
begin
dbms_output.put_line(to_char(SYSDATE, 'HH24:MI:SS'));
FOR j IN 0..999999
LOOP
SELECT COUNT(*) INTO i FROM DUAL;
END LOOP;
dbms_output.put_line(to_char(SYSDATE, 'HH24:MI:SS'));
end;

the time is the same for PSD 11 and 12.

The problem with slow response to error in test window (different forum thread) is however still present.