From franl ...

*If non-nil, ERC displays IRC colors and other highlighting effects.

If this is set to the symbol 'remove, ERC removes all IRC colors and
highlighting effects.  When this variable is non-nil, it can cause Emacs to run
slowly on systems lacking sufficient CPU speed.  In chatty channels, or in an
emergency (message flood) it can be turned off to save processing time.  See
`erc-toggle-interpret-controls'.