It is (high) time the UN took some measures. This suggests urgency. It is the speaker's opinion that these measures should be taken immediately, and even that they should have been taken sooner. You may be confusing this with this simpler structure: It is time FOR someone to do something. e.g. It is time for me to go. This is simply a statement of fact.

You raise a fascinating question. "It is high time that..." is an unusual expression in that it works well with both the Past Subjunctive Mood and the Present Subjunctive Mood. It differs in this respect from the very similar expression "It is important that..." which really only works well with Present Subjunctive: "It is important that you fix that" (Present Subjunctive) "It is high time that you fixed that" (Past Subjunctive) "It is high time that you fix that" (Present Subjunctive) With "it is high time that...", use Past for something that should have but hasn't happened. Use Present if you do not wish to convey a sense of urgency or lateness.

It could be “takes”. But I think this is an example of “distancing” so past tense is used. It is unlikely the UN will take measures.