Rakuten Securities shows the performance of the fund (blue line) compared to the average of its category (分類平均, red line). And so far this fund seems to have been quite a bit underwhelming.
For the 3 last months:

6 last months:

1 year:

Since inception:

For perspective, since inception the fund is up by 33.68%, whereas the category average is +50.59.
For comparison, here is the 1 year performance of eMAXIS Slim 先進国株式インデックス:

Here we can see that the fund is much close to the average (and even beats it by 10% since inception), which one would expect due to its low fees.
Any idea why our champion fund seems to be performing so "poorly" despite being low fees and very diversified?