Thanks. Fortunately I got the Pro (not the Plus) card.
I don’t find it wird that random write speed is much slower than sequential write speed on flash cards.
In a flash card you cannot just write e.g. 2 bytes. You have to erase a cell and rewrite everything. This is also the reason it is very important to format the card using a tool made for flash cards (like SD Memory Card Formatter | SD Association) and not just the tool that comes with the operating system. Wrong formatting will not only make the card slower but it will also wear much faster.