At which part of the day do you perform the least well at work (eg: morning, afternoon...) and why?

Same all the day. Plant output must be same/consistant each hr.

For me, sure noon time; because the morning time usually is spent to support my team and contributing in meetings of top management or team meeting.

So noon time usually the right time to perform the least well after releasing the stress of daily important issues.

Afternoon, because brain and body wants little rest .  

At the morning because my brain is fresh & no pressure affect it yet 

Morning since energy is fresh and energize.

Between to Hrs are too touch to deal with sensitive issues & I would say slow reflexes for one hour bit sleepy too :-) 

I would probably say morning, because I am fully aware and active  and uptodate to what I have to do as in tasks, etc.

Morning, when my brain is fresh and active to work