OK, that gives something to go on. Probably need to break out a debugger now. Not sure if I'll have time but if I do, I'll have a look.
I would suggest raising an issue on GitHub. Perhaps the author can work some magic and speed things up or at least indicate why it is slow.
On my own instances of Node-RED however, it doesn't seem particularly slow & I'm using polling as well. I wonder if you have anything odd going on at the network level.
Hmm, what country are you in? Do you have a different Internet connection you could try (maybe tethered mobile?) My paranoid mind is making me wonder if something is interfering with your connection to Telegram.
On a less paranoid track - what is the state of your Internet connection?
It is indeed more complex and you have to have an inbound connection through your firewall so taking care of security would be important.
However, it is certainly a lot more efficient and messages will generally get from Telegram to Node-RED bit faster too and it will be less of a load on your network.