If you use 3.9 as you would use 3.1, it's not slow. Conversely, if you add cosmetics to 3.1 to reach the level of a bells-and-whistles 3.9 installation, it will be just as slow.
It seems people nowadays often jump from a bare bones 3.1 system to a spiced-up 3.9 system, so naturally it will seem slow. But when OS 3.9 was released, the typical jump was from a 3.1 system running NewIcons, a JPEG backdrop, a TCP/IP stack and a WBStartup drawer filled to the brim.
The main disadvantage of 3.9 is the reboot on cold boot, adding to the startup time.
Last edited by idrougge; 17 June 2016 at 14:52.