Normally a lookup will be in cached form by default. Which means that when we do lookup on table then Informatica will be go into the lookup table and store the data in Cache file, which avoids re lookup into the table when we need the data again.Informatica will make use of the cache file and this makes the lookup more faster.
But now the question comes why we need Persistent Cache in Lookup. To use Persistent Cache we need to check the option of Using Persistent Cache in the lookup. When we do that what Informatica does is that it will store the cache file and won’t delete it after run of the session or workflow.
This becomes handy for situations where we use the same lookup in many mappings. Suppose that we use the Lookup LKP_GET_VALUE with same lookup condition and return and output ports in 10 different mappings. In this case if you don’t use Persistent Cache then we have to lookup the table 10 times, and if the table is a huge value then it will take some time to build the cache. This can be avoided by using Persistent Cache.