Where to look for the reason for this behavior of the database server?

Question

MySQL

When a query changes any data related to the index, the execution time sometimes increases several times, and, worst of all, all other queries (even if they do not concern this table in any way) are executed only after its completion.
How can this situation be influenced and / or where to look for the cause? If you need any specific information, I will write.
UPD: checked, this does not always apply to indexes. Sometimes it happens even when changing data that is not in the index.
UPD2: requests stop at the updating stage.
UPD3: the load on the processor in the normal state is 20-40%, during sticking - 100%. Memory is always used by 50-60 percent and almost never changes.

Answer 1

The first thing to check is the table type.
MyISAM or InnoDB.
The second presence of a transaction in requests.
The behavior is typical for tables of the first type. They have "table level lock" for changes in the transaction.
Those. the whole table is locked