This is the current limit for me (Winnable times)
1.
Chopping - up to 10 malus(12 with perfect trees line up)
2.
Sickling - up to 6 malus
3.
Mining - up to 10 malus(12 with perfect rocks line up. Happened once the 12 rocks are next to each other on 3 rows + 2 on their top)
4.
Watering - up to 10 (maybe 11) malus (12 always gave me blooms just when timer runs out. If I go any faster the plant will start bugging out aka plant not being watered when the action is seen and the square is perfectly on plant)
5.
Fighting - Can't win even with no malus (8/9 best record so far. Archers like to elude me and avoid where I am approaching making me chase and then Archers on the ground start adding up and ganging on me)
6.
Picking - Only when no malus (Can't do even with 4 malus.. 2 malus maybe still can squeeze through? Never tried)
7.
Hoeing - up to 8 malus (Can't move-click faster or the naked look-like boy will start swinging the air instead of the empty ground where it is yet hoed)
So given my stats you can see why with 4 malus per key I can only win 3 and if very lucky 4 keys. At 6 malus per key usually 2.
PS. So the most winnable challenges are Chopping, Mining and Watering (For me at least) this those are the challenges I put more effort to try and improve. Archers too but they are really unpredictable..