[MPS test] Is there some customization options for `scratch/igc` feature?

[Eval Exec <execvy@gmail.com>]

Hello,

I'm helping to test `scratch/igc` branch (commit id: 42731228d24) .

At start, my cursor is in buffer's top line, I long press [down] key
and don't release, expect the cursor will move to bottom line. I feel
lag, and buffer window doesn't update until I release [down] key.
Then I see there is some message in `*Message*`:
```
Garbage collecting...
Generation 0 of a chain has reached capacity: start a minor collection.
Garbage collecting...
Generation 0 of a chain has reached capacity: start a minor collection.
Garbage collecting...
Generation 0 of a chain has reached capacity: start a minor collection.
Garbage collecting...
Generation 0 of a chain has reached capacity: start a minor collection.
Garbage collecting...
(many...)
```

What's the `capacity`? how can I config the `capacity`? I think the
lag is related to the `capacity`
I tried `(customize-group 'mps) or `(customze-group 'igc)` but look
like igc feature doesn't provide customization options.