In a simple table with no complex indexes, does a large number of existing rows slow down an INSERT?
When you are mass INSERTs into an InnoDB table with just a Primary Key you are building up the gen_clust_index (a.k.a. Clustered Index).

Secondary Indexes are built in conjunction with the Clustered Index. Since you have no Secondary Indexes, this reload would be just fine.

I would high recommend doing two things when reloading:


Organize your INSERTs as extended INSERTs
Set bulk_insert_buffer_size to 512M or 1G


When you use the bulk insert buffer, it helps load the data into a treelike structure that makes it easier to load into a non-empty table.