I don't know why i'm doing this, riot is going to ignore it like almost everything else.

It's simple, the game is reading some file from HDD, when the 1st voidling is cast. It's very easy to see this with ProcessMonitor, especially if all sounds are disabled to hide the HDD access for the audio files.

The 1st tower shot also causes the game to read something from HDD.

Some announcement, most likely "Welcome to Summoner's Rift"

AND

Opening the Shop,
where i get Bug Splats from time to time. Now i think it's related to team mates moving or using skills in that moment, causing the game to load the matching sound for this and the shop files.

bump

Ok, i didn't check this for HDD access, but buying the 1st ward also lags the game. This caused me to fail buy 1-2 times before i started to wait a little after buying my 1st ward.