I did get around to running the replay.
-Replay ends when HQ is destroyed. No Dooms continue to spawn and no weird behavior seen with the transports.
-I have a i7-12700H (gets a CPU score under 100), replay speed reached 0 at 42 mins, then fluctuated at +1/+2 afterwards. Once 2nd HQ was destroyed, speeds went up to +2/+3. Checking Task Manager, CPU use for Supreme Commander never went above 20%.
Unfortunately, Lua does not support multithreading, so everything has to be done in sequence. The more units = more slow down. There was always a sudden drop in speed whenever ASF waves would spawn, which would continue from all the projectiles fired at the ASF swarms, then sim speed would go back up once the swarm was dead. Did you adjust settings to have large ASF Response waves now, when you host?
Large ASF swarms on Dual Gap always caused significant speed reductions for me on my older laptop, which had a CPU that scored around 220, but was closer to 250 when it started to get throttled. Don't know why air, particularly ASFs, seem to drop sim speeds so much. I'm guessing their high speed requires frequent calculation updates from the CPU, and since Supreme Commander doesn't really support multicores, this really slows things down.
Navy appears to still be spawning off map. Though this was only around 30 ships, not really enough to cause much issue. I'll have to figure out my error. Looked to only be affecting the X axis and not the Z.