problem is table design and write amplification. Every row insert triggers update into every index, so you get classic amplification problem.

Separate your table into Cold (with all indexes and bells and whistles) and Hot (heap table with no indexes except PK).

Insert as many rows as you want into Hot heap, and then move them in the background into cold in batches, so that index recalculation is amortized across many rows, instead of per-row.

Another poster suggested partitioning, thats the same idea: separate Hot and Cold data into partitions and keep hot partition as heap