What does slow down an InnoDB database that is fixed by re-importing it?
Performance degradation was most likely caused by your table getting more and more fragmented by inserting and deleting many rows every day.  
Dumping your table, dropping it, and reloading after many deletes will compact the space on disk and reduce the size of the .ibd file, assuming that it uses its own tablespace.  


You could accomplish the same thing with a null alter, like this:

ALTER TABLE sometable FORCE;


which will rebuild the table.