Why did Minerva McGonagall suggest that Hogwarts only be given basic protections?

Question

Why did Minerva McGonagall suggest that Hogwarts only be guarded/protected basically, and not to their best efforts?

Here's a line from Deathly Hallows, just shortly after Severus was chased away by the combined effort of McGonagall, Flitwick, and Sprout:

‘Professor, we’ve got to barricade the school, he’s coming now!’

‘Very well. He Who Must Not Be Named is coming,’ she told the other teachers. Sprout and Flitwick gasped; Slughorn let out a low groan. ‘Potter has work to do in the castle on Dumbledore’s orders. We need to put in place every protection of which we are capable, while Potter does what he needs to do.’

‘You realise, of course, that nothing we do will be able to keep out You-Know-Who indefinitely?’ squeaked Flitwick.

‘But we can hold him up,’ said Professor Sprout.

‘Thank you, Pomona,’ said Professor McGonagall, and between the two witches there passed a look of grim understanding. ‘I suggest we establish basic protection around the place, then gather our students and meet in the Great Hall. Most must be evacuated, though if any of those who are over-age wish to stay and fight, I think they ought to be given the chance.’

‘Agreed,’ said Professor Sprout, already hurrying towards the door. ‘I shall meet you in the Great Hall in twenty minutes with my house.’

Why did she only suggest basic protection around the place? Did she forget that Harry needed "as much time as possible," which she said clearly just a few lines earlier? Wouldn't it be more useful to have a barrier that will last longer so Harry will have more time?

Answer 1

As suggested in the comments, the basic protection was a first, interim step that can be done in parallel with gathering, informing, and evacuating the students. After this, the heavier duty protection can then be put in place to build on the basic foundation. There is a line referencing this in the following chapter (Deathly Hallows, Chapter 31):

‘We have already placed protection around the castle,’ Professor McGonagall was saying, ‘but it is unlikely to hold for very long unless we reinforce it. I must ask you, therefore, to move quickly and calmly, and do as your prefects –’

McGonagall confirms that some protection is in place, but that it is not the best that they can provide - and the intent is to go back and reinforce the protection. This may include further enchantments, but a large part appears to involve putting witches and wizards in place to directly fight and repel Voldemort's forces.

Professors Flitwick, Sprout and McGonagall are going to take groups of fighters up to the three highest Towers – Ravenclaw, Astronomy and Gryffindor – where they’ll have a good overview, excellent positions from which to work spells. Meanwhile, Remus,’ he indicated Lupin, ‘Arthur,’ he pointed towards Mr Weasley, sitting at the Gryffindor table, ‘and I will take groups into the grounds. We’ll need somebody to organise defence of the entrances