Simply there are two differend Types of TileEntities.
First one: That tick. That produces lag.. How much depends on the function. (Can be a Furnace, Chest, Sign, Hopper... Basicly every TileEntity in Vannilla minecraft (yeah i checkt that)),
Second One: TileEntities that do not tick and act like a DataStorage. (Can be IC2 Cables, Decorative blocks (in most cases)... Well i do not know much more)
The thing between ticking TileEntities and non Ticking TileEntities is that Ticking TileEntities are in a list (which depends on the dimension) and get called every tick. But NoneTick TileEntities are not staying in that list and do not get called at all. (Note even if the ticking TileEntity Does nothing it creates lag)
So if you have 10k TIleEntities and 95% of it are None Ticking tile Entities then you do not have to care really much because they all act like Normal blocks. The Only thing you will have is that your Internet connection will be used more if you play on the server. And that even only if a chunk get reloaded. (which is not that often. Depends how much you move)
Thank you very much.