Investigate FeedStore subscription performance in storybook

[dboreham]

Reproduce:

1. Run storybook in incubator.
2. Create 5 feeds.
3. Add 20 messages (blocks) to each feed.
4. Create a subscription.
5. Observe that the aggregate blocks in the subscription take order(second) to render.

Determine why this operation seems to be so slow given that reading a message from a feed is believed to take order(microseconds).

[tinchoz49]

Related work:

• Add test and improve performance random-access-storage/random-access-chrome-file#4
• Add batch support in createReadStream holepunchto/hypercore#261
• https://github.com/dxos/random-access-multi-storage/blob/master/benchmark.js
• https://github.com/dxos/browser-runner