What makes a game lag?

I recently noticed that you could slap tile sets together with photo shop, for example placing 001grasslands01 together with 003woods01 and 004forest01 to make a nice tileset..

I was wondering if something like this would lag the game, (having a big tileset, some of mine are 10X as big as rtp) or is it just events??

only events/

having a WAAAAAAAAAAAAY to overlarged tileset also laggs...
I think..

I don't think a big tileset will cause the game to lag, but I know it can cause the game to hang when transferring to the map with the large tileset.
Events can.
Scripts can.

Pretty much anything which takes processing time can cause the game to lag. 

Events.
Scripts.
Highly done Graphics (huge maps etc)

Highly done Graphics (huge maps etc)

Now fully correct. Many big graphics can, but mainly a big map has the 3 layers as 3 huge sprites. Just good rendered graphics alone can't make the game lag.