Even when I had this in the hot path 10 years ago and was owning hundreds of objects in a particle filter, handing out ownership copies and creating new ones ended up taking ~5% (ie making it a contiguous vector without any shared_ptr). It can be expensive but in those case you probably shouldn’t be using shared_ptr.

Oh, and the cost of incrementing an integer by itself (non atomically) is stupid fast. Like you can do a billion of them per second. The CPU doesn’t actually write that immediately to RAM and you’re not putting a huge amount of extra cache pressure vs all the other things your program is doing normally.