as I did not notice any significant hitches it took around a third of an hour for the 1x test and close to half an hour for the 3x test

Not having any hitches certainly doesn't show that at 1x speed you're getting the same framerate as at 3x speed. I'll explain a bit more about frame rate to clarify why measuring real time makes no sense at all.

At normal speed, the graphics are updates 60 times per seconds. The number of graphic updates per seconds (in other words frames per second - FPS) is exactly your framerate. The game also updates the game state 60 times per seconds. Game state updates are called "ticks", and at normal speed there are 60 ticks per second. If you increase the game speed, you effectively increase the number of ticks per frame. So if everything goes well, increasing the game speed to 3x, will result in 60 frames per second, and 3*60=180 ticks per second.

However, most computers aren't able to handle this amount of ticks, especially during late game when there are a lot of updates that need to take place and with a lot of mods. It can certainly be possible that you get 60 FPS at x1 speed (and 60 ticks per second), but only 30 FPS at x3 speed (and 90 ticks per second). In this case, running at x3 speed, in practice only lets your game run 1.5x as fast.

So everything in game is updated using ticks (including the in-game time, one hour in-game is equal to 2500 ticks), and a bill takes a certain amount of ticks. This number of ticks is calculated by the pawn's skill and health, the material, the bill itself, room temperature, and some other factors. If, taking into account all these factors a bill would take 5000 ticks, it would take exactly 2 hours in-game.

Because of all this, measuring real time effectively only measures how many ticks per second you're getting, which only tells how well your computer is capable of running the game at higher speeds and certainly won't measure how fast the bill will be completed in-game, since real time has no relation with in-game time. Since jobs always take a fixed amount of ticks (if no conditions change that affect the work speed), and ticks are never skipped, we can be certain jobs always take the same amount of in-game time.

You're of course free to measure if this is true, but like I explained, measuring this would only make sense with in-game time and with long enough experiments.

it took around a third of an hour for the 1x test and close to half an hour for the 3x test.

This is a way too short time interval to do any valid comparisons. I recommend, like Canute did, to do experiments with very long tasks, that take at least 10 in-game hours to complete. But again, such experiments won't yield any surprising results.