Lines 9-12 Summary

Get out the microscope, because we’re going through this poem line-by-line.

When Ajax strives, some Rock's vast Weight to throw,
The Line too labours, and the Words move slow;

Ajax is a Greek hero in Homer's Iliad. All of the prim and proper readers in Pope's day would know about him.
Ajax actually throws a rock during one of his fights with Hector during the Trojan War. That's what this line seems to be referring to. It doesn't matter so much for this poem that we know exactly what happens with Ajax, but that we know that Homer is an amazing poet and our speaker is using Homer's writing as an example of sound and sense.
The word "Rock's" is hard to say just like a heavy weight is hard to throw. The process of saying "Rock's" makes you strive over the word "rock" just like Ajax would strive to lift it.
The meter causes the laboring of line 10. One extra stress at the beginning and three stresses in the row at the end of the line make us trudge through each word: the LINE TOO LAbors, and the WORDS MOVE SLOW. Exactly.

Not so, when swift Camilla scours the Plain,
Flies o'er th'unbending Corn, and skims along the Main.

Camilla is another shout-out to Greek and Roman mythology. She is the virgin queen of the Volscians.
The great Roman poet Virgil writes that she was so swift she could run over a field of corn without bending one blade and run over the sea without getting her feet wet. Notice that the speaker kind of steals Virgil's way of describing Camilla's swiftness. By paraphrasing Virgil, he's implying that he's in the same league as this great poet.
The speaker is also doing the amazing things that Camilla does except with his verse. He creates swift lines by using iambic meter. Also, while Camilla flies over the corn, the poet flies over two syllables by changing the word "over" to "o'er" and combining "the" with "unbending" by taking out the "e."
The second line contains more syllables than the others. The comma in this line cuts it in half: the second half moves swiftly in iambic meter while the first half skips syllables with the elided words "o'er" and "th'" ("elided" just means taking out a syllable) and then extends the line extra syllables with the three-syllable word "unbending." This makes sense because the poet does not bend the word to match the expected number of syllables per line.
Whew! He's doing a lot here.