A logon in PL/SQL Developer takes a very long time. When I subsequently execute a SQL Window or Text Window, this also causes a huge delay.

This seems like a connection problem. Each SQL Window and Test Window uses its own session, and therefore needs to connect when executed for the first time. You should check this file:

%OracleHome%\Network\Admin\sqlnet.ora

If it contains this line:

SQLNET.AUTHENTICATION_SERVICES=(NTS)

Change it to a comment line:

# SQLNET.AUTHENTICATION_SERVICES=(NTS)

This is a known Oracle Installer bug that can cause unacceptable connection delays.