In GOG release the console should be enabled by default. So skip this step if the ` key (above tab) works for you. If not then:
Step #1 - Enabling console
Open hitman.ini in notepad
In the section "Enable Console" add enableconsole 1 as a separate line. Save the file.
Step #2 - setting timemultiplier
In game, press ` to bring up the console. Type in IP_TIMEMULTIPLIER .
Now, is in between of 1-99, you have to experiment to find the right setting. But .99 is usually default, so try around .70 and go from there.