I love this. The speaker is growing impatient to finally be relieved for the day. As you want time to move faster, it then moves a lot slower. This is relatable for every person who reads it.

Time slows and I sigh

^That line made me laugh. It tastes like so much frustration is built up, and then the sigh is just funny. It might not be intended to be, but it gives the poem that impatient feel. I like it.

Half of an hour left
The clock now ticks more loudly
Time slows and I sigh

The 2nd lines reveals as though it's the time for an explosion of an alarmed bomb. That sigh is the one before the life ending. That's really funny.

Time does seem to slow, when you're waiting for something to be over, doesn't it? It's so simple and happens everyday, but reading about it, in this forms makes it seem, somewhat, thought-provoking. Well done.

Now this senyru I found funny, specially the last line.
Time seems to slow down the most when we want it to go faster, but it really isn't time that slows down. It's our perception of it.

I like that line " I sigh" like you are saying " I have nothing else to do but to wait"