Game started lagging

Hello
I’ve started creating the game few weeks ago. Recently I started experiencing lag while building the game. I’m aware that the map became more complex and used many hi-res textures and materials.
My question is: How can I optimise the game? Should I partition my map into levels? If so, at which point should I create a new level for the continuation of the map?
I know this might be a silly question but I still wanted to hear some ideas and discussions about game optimisation in general.
Ask for additional information if required.
Thanks

Yes, exactly.

So by lagging you mean descresed preformance?