Do large longtext necessarily slow down a database
On MyISAM tables, fragmentation can easily occur where rows have variable length and deletions are frequent. This can affect index reads and full table scan performance by increasing the number of pages that need to be read to find the data, and preventing the use of sequential read-ahead technology.

See this Serverfault question for good info on fragmentation, how to detect it and react to it: https://serverfault.com/questions/202000/how-find-and-fix-fragmented-mysql-tables

This is less applicable on InnoDB, where large objects are stored separately to the rest of the rows, and tables are index-oriented.