Just because you shift around worktimes IMHO dont mean your 'incompetent' / are underperforming.
Offcourse if you simply cannot get the job done in the allowed time, that is another matter... Still that can depend on the source of "overrun", other projects/priorities may have gotten in the way?
No way is a 1:1 between working non-allowed overtime: underperfoming
Also some employers just expect you to, not to long ago I was expected to not declare any overtime and/or shift hours around... 9-17 thats it... be in by 9 leave a 17. Offcourse some manager would come 16:55 with a job for an hour, if I could finish that before going home and that he does realize I would shift the extra time to Friday... Then Friday lunch time, I leave... But that is not allowed, so I worked to 17.
Next monday again some problem pops up and I leave at 17, next day all hell in the office "why didnt you finish my request"?
Flexibility is a 2 way street! Companies/managers expect the world for a penny. They expect commitment and loyalty but in return give nothing

A dutch proverb say: Veel beloven weinig geven doet een gek in vreugde leven
Roughly translated: Promissing a lot, doint nothing makes a crazy man happy (Crazy man = Managers

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