Thanks again for your reply
As I'm currently in another country, I just found that their internet is ADSL, different from my country.
The place where I currently test uses ADSL, whose upload speed is only around 35Mbps. So the speed of wireguard server on the peer over the internet is restricted to 35Mbps, because vpn traffic both uploads and downloads in terms of vpn server.
Therefore, I just changed the test environment, which set the wireguard server on the intranet with the wireguard client, then the wireguard speed is about 87Mbps whose speed is still restricted by old test router wan 100Mbps speed.
The test environment is currently restricted, I can't test further for now.
But I expect that wireguard speed on my device will be higher than 87Mbps hopefully over 100Mbps, if use new router later.