tails-upgrade-frontend causes very high CPU load when downloading upgrade

When running Tails 4.22 from USB and downloading the upgrade to 4.23, tails-upgrade-frontend causes very high CPU load. On the machine I'm running now it's 50%, which corresponds to CPU1 at 100% on a dual core machine (according to gnome-system-monitor).

Current test setup:

• SanDisk USB 3.2 stick with 64 GB (reasonably fast), Tails 4.22 correctly installed on it, no persistent volume
• Thinkpad X230, Skulls (Coreboot + SeaBIOS), dual core i5 CPU, 8 GB memory, ethernet connection

The test setup is not particularly important though, as other combinations of stick + laptop show the same behavior.

Preparation: USB stick is plugged into an USB 3 port. Machine is booted to Tails, network/Tor connection is being established.

Expected behavior: update is being triggered, download should be moderately fast (around 500 KB/s, obviously depending on circuit), CPU load stays low, at least during the downloading part.

Observed behavior: update is being triggered, download is very slow (around 10 KB/s) and the CPU load goes up (one core at 100%).

Important: this is not the first time I've observed this behavior, it's been like this with previous version upgrades too. It has now happened often enough that it's time to file an issue.

Any idea what could cause this?