Unusably slow Bridged network upload after upgrading host to Tahoe

**Describe the issue**

When running macOS 15 Sequoia, network performance inside macOS VMs was roughly equivalent to the host's performance. After upgrading the host to macOS 26 Tahoe, the upload side of network connections has become much slower and often unreliable.

I first noticed this because a VM that was running GitLab CI jobs was timing out when sending 150MB files back to the GitLab server. The Activity Monitor's Network tab showed that upload speed was maxing out at about 250Kbps. I confirmed that the same upload process run from the host to the GitLab server is multiple Mbps (it finished too fast for me to see the result in the Activity Monitor.)

I tested with VMs running Sequoia and Tahoe, and I see the same problem in both of them.

I normally run these VMs in Bridged Network mode, so I can connect to them with VNC from another computer on the same local network, and I observed that the VNC drawing is incredibly slow, which is consistent with very poor upload speed. When I switch them to Shared Network mode, the network speed is normal, but then I cannot access them from my other machines.

**Configuration**
* UTM Version: 4.7.4 (originally seen on 4.6.3, upgraded hoping for a fix)
* macOS Version: 26
* Mac Chip (Intel, M1, ...): M4

**Crash log**

N/A

**Debug log**

There is nothing in my system log other than generic "ASL Sender Statistics" info messages.

**Upload VM**

This happens with all VMs that use Bridged Networking. I created a plain VM with no changes and copied in a large file to test with, and it did the same thing.