Serialization: good or bad in a multi-thread environment to maintain a cache?

Which is a better approach : using serialization to create a cache or simply writing objects values in the form of string arrays to a file using delimiters? As I recently was told that serialization makes a different thread to process and should not be used in a multi-threaded environment/application where there are timing constraints and you need faster processing and less interruption! So Is it true, that serialization is in efficient in a big multi-threaded java project ?