Suggestions:
1. Complain earlier, if you've been playing for a long time, maybe some frustration could have been avoided.
2. The core game loop hasn't changed architectures since it was written 30-some years ago. In short that means it is optimized for memory constrained machines and is single-threaded so your umpteen cores and next to useless except to keep the game on a core that isn't heavily used by other processes on your machine.
3. If you want it changed, I'll need you to invest a lot of sweat equity (or monetary equity but nobody has jumped on that bandwagon as yet!). Harpoon Classic/HC/HCE/HUCE is community developed. I've been the only programmer since around 2009 and was the only programmer from 2005-2007 or so. It is a hobby for me and gets way less time than I'd like. We're very thinly manned right now with me doing coding, CV32 making sure the forum isn't overrun with articles & spammers, and eeustice making almost every issue report and doing almost all of the testing that gets back to me.
4. Enough with my complaining... There is a ton of potential with the game, I am capable of writing and do write multi-threaded code, there is even a little bit in the game that is toggled off since the bit I chose was a bad choice and makes the game slower rather than faster. If people step up and take an interest in making the game better, we can accomplish a lot so step up!
5. For the particular slowness you are seeing, post up the background information, name of the scenario, build of the game, zip up and attach some saved games, etc. Without 3) it is unlikely much will change in the next few years but every once in a while I spend most of a PTO day cranking out something useful for the game.