So theoretically bitcoin transactions can't be accelerated any more? When compared to altcoins that can be instant like doge or trx. Honestly, I don't know about the concept of pow, pos or anything else, I just hope that in the future bitcoin can be instant like altcoins.
I also read in the advertisement under the first forum post that bitcoin is still in beta status, does this mean that in the future bitcoin can be upgraded to be more sophisticated?
It can, but high unlikely because you need to change the protocol, vast majority won't agree with that. Reduce the block time will increase the possibility of getting attacked with double spend and 51% attack. That's why there are many people fork Bitcoin e.g. BSV, BCH, BTG etc because they want to create their own Bitcoin which they call it improvisation.I also read in the advertisement under the first forum post that bitcoin is still in beta status, does this mean that in the future bitcoin can be upgraded to be more sophisticated?
It's no longer beta status according to achow101, I believe Bitcoin will always be improved to face or prevent a threat that could harm the ecosystem from time to time.
It is no longer experimental, because the latest version is "22.0" instead of "0.22.0", the leading zero is missing 
The version number is not an indicator of experimental or beta status.Bitcoin Core still (intentionally) contains a line that says "this software is experimental". The line saying that Bitcoin Core is beta software was removed in 2014.