Sure. Just for you, I checked the times for making 5x Big Mango (200pcs with all gardening equipment).

Big Mango x5 time:

base time 60
That's the time when all speed equip is in my inventory.

Now I'll take them out 1 or more at a time:

wood bell 59
stand bell 59
windmill 59
These 3 all save about the same amount of time. 3% is less than 2 minutes, so you can't see the difference between 2% and 3%.

wm wb 57
wm sb 57
wb sb 57
With any 2 of these, you save 4% or 5%, less than 4 minutes, so again, you can't see the difference. (Something that takes over 100 minutes would be a better example, of course.)

wm wb sb 56
With all 3, you save 4 minutes (8% is less than 5 minutes). The actual time saved will be 8%, but the interface only shows full minutes.

Having already proven they are at least near accurate, I'll add in the Air Condition just for completeness:

ac 51
Hold up! 9 minutes is 15%, not 12%! This should be 53 minutes...

ac wb 50
ac sb 50
ac wm 50
ac wm wb 48
ac wm sb 48
ac wb sb 48
ac wm wb sb 47

Now, that should be 20%, but 20% would be 12 minutes, and this is 13 minutes lower. That's 22% or 23%.

Well, if there's a description error, it's in the Air Condition. I'll have to find something that takes a REALLY long time to make, to get a finer measurement.