Is there a performance hit using decimal data types (MySQL / Postgres)

Question

I understand how integer and floating point data types are stored, and I am guessing that the variable length of decimal data types means it is stored more like a string.

Answer 1

I am guessing that the variable length of decimal data types means it is stored more like a string.

Taken from MySql document here

The document says

as of MySQL 5.0.3 Values for DECIMAL columns no longer are represented as strings that require 1 byte per digit or sign character. Instead, a binary format is used that packs nine decimal digits into 4 bytes. This change to DECIMAL storage format changes the storage requirements as well. The storage requirements for the integer and fractional parts of each value are determined separately. Each multiple of nine digits requires 4 bytes, and any remaining digits require some fraction of 4 bytes.