You say very different things when you say “beaucoup de temps” and “la plupart du temps“.
Like you said it’s equivalent to “a lot of time” vs “most of the time”:
when you say “beaucoup de temps” you’re counting units of time, or rather gauging because it’s uncountable.
When you say “la plupart du temps” (or “tout le temps“), you’re expressing fractions of the time as a whole.
It’s the same difference between “a lot of sugar” and “most of the cake“. It’s completely different things you’re measuring.
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