dmesg shows no errors. Earlier I had power issues (external hdd seems to draw a bit too much for the USB PD supply). Now that I connected the Lenovo USB-C supply of my notebook, the power issue is resolved but the low speed remains.
lsusb -t:
/: Bus 08.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/1p, 5000M
/: Bus 07.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/1p, 480M
/: Bus 06.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/1p, 5000M
|__ Port 1: Dev 2, If 0, Class=Mass Storage, Driver=usb-storage, 5000M
/: Bus 05.Port 1: Dev 1, Class=root_hub, Driver=xhci-hcd/1p, 480M
/: Bus 04.Port 1: Dev 1, Class=root_hub, Driver=ohci-platform/1p, 12M
/: Bus 03.Port 1: Dev 1, Class=root_hub, Driver=ohci-platform/1p, 12M
/: Bus 02.Port 1: Dev 1, Class=root_hub, Driver=ehci-platform/1p, 480M
/: Bus 01.Port 1: Dev 1, Class=root_hub, Driver=ehci-platform/1p, 480M
lsusb:
Bus 006 Device 002: ID 0480:0900 Toshiba America Inc
Bus 006 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 005 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 008 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 007 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub