I assume that these errors are showing up within an editor within Eclipse when you have a specific file open.
a) What kind of file is it (specifically, the extension)?
b) What editor was used to open the file (Eclipse has many different editors)?
Perhaps the wrong editor was used to open the file, and thus the editor is not properly interpreting the file.
As far as the slow down, I suspect that Eclipse is either automatically building or indexing your files. You could try turning off automatic builds, which might help. Also, correcting the editor association for the file that has the errors might also help because that it causing the parser and indexer to work harder.