Partial indexes Vs standard indexes : Insert performance

Hi,

I just want to verify if I'm understanding this correctly:

I have a table in which I store log from my firewall.
For the protocol column (3 distinct values: TCP ~82%, UDP ~17%, ICMP
~1%, the table contains 1.7M rows), I use a partial index to find ICMP
packets faster.

In my understanding, a partial index is only touched when a matching row
is inserted/updated/deleted (index constraint is true), so if I create a
partial index for each protocol, I will slow down my machine as if I had
created a single "normal" index, but it will find rows faster (the
distribution is not uniform)...

Is this correct?

Thanks a lot,
--
MaXX