Is mentioned above, I'm using 3.4.2 - stable, which is the latest release build.
Decrementing an integer isn't a real world use-case, but I'd like to know why it's hurting performance anyways; and in 8+ hours of reading articles and forum threads on multithreading bottlenecks for general programming, I haven't come across any mention of overhead from heap allocation. The profiler shows minimal memory usage and expected CPU usage, as the threading maxes out all of my CPU cores.
That being said, I've swapped out the decrement function for a function that counts prime numbers within a range, which is a lot like decrementing but with a division operation and conditional statement. As far as the logic goes, it's very similar to a pathfinding algorithm (lots of conditional statements and memory allocations), which is a common function used in multithreading. The performance still suffers from multithreading, though.
I'm still learning all this myself, and I appreciate your efforts, but I think that blaming the performance hit on general overhead is leading me in the wrong direction. That is unless Godot specifically suffers from multithreading overhead.