Is it still an unplayable mess on modern computers? I remember trying to get it to run on my last rig with a physical copy I got my hands on and couldn't for the life of me get it to function in a playable manner. When I was able to get it to run, it would do so at a snails pace and freeze regularly. Has anything changed? It's a game I really want to play because of how much I love the original.

There were Windows 7 compatibility fixes and performance improvements made when higher resolution support was added for the Anthology release of the game.

A few people did still run into slowdowns (much rarer than with the disk version), but in a couple cases it was due to anti-virus programs, or an issue with the steam vent effect, and performance was fine after getting out of the starting cell block.
In the original disk release or the pre-Anthology GOG release, some people could fix performance problems by limiting the amount of RAM that Windows would use.
It may also help to set the game to run on a single core; the easiest way to do this would be to right click the div.exe program, or shortcut for the game, select Properties, switch to the Compatibility tab and set it to run in Win 9x mode.