Pathping (or just tracert) to some internet site is a good tool to check where latency is coming from (e.g. make sure it's not coming from the network and happened to coincide with upgrading the PC). If you can connect it to another wifi network, that would be another good way of making sure whether it's specific to the computer.
Maybe keep netstat up while making a bunch of connection attempts to check whether they really are waiting for the first one to close. Also, if your router firmware supports it, see how many open connections it shows coming from your computer.
Also, be sure to rule out
nachos.