Slow inserts in mariadb columnstore
Column-oriented database engines, not just MariaDB ColumnStore, are specifically optimized for read operations on large volumes of data. For this they use auxiliary data structures (like ColumnStore extent maps), query parallelism, compression etc. This comes at the cost of increased overhead for standard DML operations. This is often explicitly stated in various technical documents, like this quote from the InfiniDB technical overview:

Note that because InfiniDB is a column-oriented database, insert and delete statements will not normally run as fast on InfiniDB as they will on row-oriented databases (more blocks must be touched).

or this from the MariaDB architecture presentation (see slide 22):

DML Writes

Slow compared to other engines

INSERT is very slow compared to cpimport




Vendors often provide special tools for bulk loading data into columnar tables (e.g. the cpimport utility mentioned above) to address this problem. This fits well into the typical columnar engine use case, that of an OLAP system with (relatively) infrequent mass updates and heavy read-only workloads.
In other words, you should not judge a column-oriented database by its DML performance, because that's not what it is designed for.