In informatica while delete records from Salesforce Object my informatica job is very slow, 1 record per sec

There are many ways to improve deletes.

1. create/make sure there is an index on the key column/s. Informatica fires delete query like DELETE FROM mytable WHERE id1=? and id2=?. so if you create an index on id1 and id2,delete will be very fast.
2. remove all indexes and keep only one index on above columns mentioned #1. Once load is complete, pls recreate those indexes.
3. if the table is really huge and you can not create index, you can create a temp index before the load and drop it after data load completes. But if table is huge this can take a lot of time to execute.

You can make use of Composite REST api to invoke multiple Rest API requests in a single REST call which can improve performance.

https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/resources_composite_composite.htm