យឺត

It's an adjective, but in Khmer, adjectives often function like verbs (stative verbs). You don't need the word 'to be'. You can just say 'Knhom yiet' (I late).

Say 'Somsos knhom mok yiet' (Sorry I come late).

Yes, 'Computer yiet' is very common.

The opposite is 'loeun' (លឿន), which means fast.

Only in very formal speeches or if you are trying to be funny by sounding overly dramatic.

In a temporal sense, yes. But for physical position (behind a house), use 'khang kroy'.

Yes, it's the standard way to ask them to slow down for safety.

The word itself isn't rude, but being late can be, depending on the context.

Use 'yiet pel' or 'yiet haey'.

Yes, 'monus yiet-yiet' describes someone who is laid back or moves slowly.