Slow performance with Samsung T5 SSD

I'm running the Ubuntu 18.04 image on a Pi 4 with 4 GB or RAM. I added a USB SSD (Samsung Portable SSD T5) and changed cmdline.txt to boot from the SSD. That is working, but when I run the speedtests, I get a score of only ~2900. This is the output from lsusb: Bus 001 Device 005: ID 04e8:61f5 Samsung Electronics Co., Ltd

And when I check with df -h, I see the ~500 GB partition mounted as root:

ubuntu@cjkohler-pi21:~$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/root       459G   16G  424G   4% /
devtmpfs        1.8G     0  1.8G   0% /dev
tmpfs           1.9G     0  1.9G   0% /dev/shm
tmpfs           1.9G  584K  1.9G   1% /run
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
/dev/sda1       253M   42M  211M  17% /boot/firmware
tmpfs           381M     0  381M   0% /run/user/1000

Here is what blkid shows, I wonder if the fact that the labels and PARTUUID are the same between the SD and the SSD is causing a problem:

ubuntu@cjkohler-pi21:~$ blkid
/dev/mmcblk0p1: LABEL="system-boot" UUID="85F3-CC2B" TYPE="vfat" PARTUUID="da84cd12-01"
/dev/mmcblk0p2: LABEL="writable" UUID="b96d647f-679e-4582-87d0-2d8c7afc91f4" TYPE="ext4" PARTUUID="da84cd12-02"
/dev/sda1: LABEL="system-boot" UUID="85F3-CC2B" TYPE="vfat" PARTUUID="da84cd12-01"
/dev/sda2: LABEL="writable" UUID="b96d647f-679e-4582-87d0-2d8c7afc91f4" TYPE="ext4" PARTUUID="da84cd12-02"

Any idea why the speedtest is giving such a low score?

Thanks

TheRemote / Ubuntu-Server-raspi4-unofficial

Slow performance with Samsung T5 SSD #125