I believe the quality of Mullvad’s Japan servers (Tokyo and Osaka) has been very poor recently.
As you can see by running ping, there is constant packet loss.
I have stopped using Mullvad because it is useless.
As you can see by running ping, there is constant packet loss.
To be fair you need to run a proper MTR to troubleshoot cases like these.
I use Japan servers exclusively and have not noticed any issues.
I never use Osaka because they’re too unreliable for speed despite being physically closer. Tokyo is better.
Where do you live? Using Japanese servers from places like Europe / US usually involves a couple of choke points net-wise.
Try running:
mtr jp-osa-wg-004.relays.mullvad.net (or whatever server you prefer) and have a look at where the problem is.
For me, living in northern Europe everything is fine until T3 is involved and adding 83% packet loss and about 250ms when the traffic is leaving Copenhagen on some peering point. After that it looks just fine.
Not really anything Mullvad can do about all the bad routing/peer on the internet.
I’m not saying it’s mullvad, it could be, but it’s waaay more likely that your ISP is buying bandwidth from someone who is buying bandwidth from someone who is peering with someone with packet loss.
Changing your ISP could help, changing vpn provider could too, it’s a hit and miss situation 
Go wild on looking glasses if you really want to figure it out.
Agreed, same can be said for New Zealand servers, very disappointed