Is std::vector thread safe ?

The discussion pointed out by Richard is nothing more than a complicated way to say
"no: no locking is done on the collection and on elements themselves", as you can (not easily, because of lot of macros and definitions that obfuscate the real code) see by looking the source code. There is nothing protecting against threads conflicts.

The different timing observed is not due to "locking" but to the impossibility to manage all the iteration into the 1st level cache of a core: if the various threads are hosted on different cores, a RAM access is required for every single write, an every subsequent read.

Although other solutions are goods, I will just provide some analogies and hints that might help understand how safe it is...

Safety would be very similar to what you would expected from a dynamically allocated C array.

Knowing which operations can invalidate iterators would also help to understand what would be safe to do.