Why do MySQL InnoDB inserts / updates on large tables get very slow when there are a few indexes?

Question

We have a series of tables that have grown organically to several million rows, in production doing an insert or update can take up to two seconds. However if I dump the tab

Answer 1

track down the in use my.ini and increase the key_buffer_size I had a 1.5GB table with a large key where the Queries per second (all writes) were down to 17. I found it strange that the in the administration panel (while the table was locked for writing to speed up the process) it was doing 200 InnoDB reads per second to 24 writes per second.

It was forced to read the index table off disk. I changed the key_buffer_size from 8M to 128M and the performance jumped to 150 queries per second completed and only had to perform 61 reads to get 240 writes. (after restart)