There could be a few reasons for this. Thanks so much for sharing your URL so I didn’t have to ask to investigate 
What I see is that your DNS is setup well, to use our CDN with maximal effectiveness:
$ host 1kohei1.com
1kohei1.com has address 104.248.78.24
…and that’s the number one “speed problem” that people have - using an “A” record for their site’s custom domain. You use our DNS hosting so all of the records we create are the “good kind”, so…that’s not your problem This article mentions a bit more details about the situation:
Just wanted to mention it for the next person who finds this from google, since that will likely be their problem
So, on to what could be your problem! Our CDN caches opportunistically, and on the free and self-serve tiers (Pro, but not Business from Netlify Pricing and Plans), you’re competing for a space in our CDN cache on EACH NODE with over 10 million other sites. So, your content may not load quickly each time, since it is not in cache and has to be fetched from our backing store in San Francisco, USA, and then is cached on the local CDN node.
I took a quick look at your traffic patterns and your site definitely isn’t loaded constantly, so it is likely that the content is usually not cached when loading your site I think the connection time vs full download shows this situation well on the graph at sucuri:
https://performance.sucuri.net/domain/1kohei1.com
While I was checking out your access pattern, I did a quick query for assets taking >2 seconds to load and there were a few dozen (66), out of over 100 thousand (specifically 101,198) in the past week, so yes, it can happen, but I don’t think it is common - at least not as far as us sending the file from our infrastructure, which is the timing we record. Since Google Analytics records from a client point of view, slow network and slow DNS can also factor in to the transaction time recorded, and there’s not really much we can do about them from our side
Finally, the nearest CDN node to Japan is in Singapore, so there is a bit of transit time to you, but we wouldn’t expect the average to be 3 seconds unless the content is not cached when you request it.
If you can capture a slow load in progress, with your browser dev tools open, we’ll be best able to debug what happened on THAT PAGELOAD if you can send us the x-nf-request-id HTTP response header value.
