Didn't work on it for a while, but now there's news! I rewrote parts of code to use vertex buffers and index arrays for almost any geometry.
Rendering is super smooth and fast now :)

PLUS: Thanks to some awesome little library I've found, I wrote a model converter for my engine, which is able to convert 3DS, Obj, Collada, Quake, Doom3, HL2 files etc. to the engines own model format.