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.