I speak English natively. I read to 1400 without difficulty, read 1400 and 1300 with some sruggle, and found beyond that it was largely unintelligible; I can understand maybe 1 in 3 words.

1200 looks harder than it is because of the change in pronouns.

...Nor shall I never it forget, not while I live!

... and that was a wife [= woman], strong and [stith]! She came in among the evil men and me [nerede] from their hands.

She slew the heathen men that me pinned, slew them and felled them to the ground. There was blood and [bale] enough. And they fell [and] lay still, for they [] might no more stand. And the Master, the [wraþþe] Master, he flew away in the darkness and was seen no more.

I said [to] her, "I thank thee, [leove] wife, for thou hast me [ineredd] from death and from all mine [ifoan]!"

Interestingly, nerede/ineredd has no descendant in modern English, but it's not difficult to understand in the passage, while leove and ifoan do have descendants, and in the case of ifoan the meaning hasn't changed, but they are harder to read.

In 1100 the idea of "just substitute the modern word in for the old word" starts to break down.