how delete/insert/update affects select performace?

Hi!

I have a table not too big but with aprox. 5 millions of rows, this table
must have 300 to 400 select per second. But also must have 10~20
delete/insert/update per second.

So, I need to know if the insert/delete/update really affect the select
performance and how to deal with it.

The table structure is very simple:

account_id integer (PK)

service_id integer (PK)

enabled char(1)

The index created on this has the same 3 columns.

Most of time the table has more insert or delete than update, when update
occur the column changed is enabled;

Thanks!