I have Mathematica 11.3 installed on two PCs. My home PC runs Linux and does not have a problem. On my office PC (Windows 7) I find that on start up, pressing F1 to get context sensitive help is initially unresponsive. After some delay, it then starts behaving normally.
Any suggestions would be very welcome.
I note the following:
The firewall slows, but doesn't totally prevent access to the internet. I thought that I might be suffering from a timeout in waiting for a response from a Wolfram server, but the problem does not go away if I select the option to disable internet access.
The virus checker probably does on-access checks on the first attempt to read any file.
The delay after pressing F1 might be of the order of 30 s, during which period Mathematica is unresponsive. I don't think help pops up at the end of this period, but I can then get help by pressing F1 again.
There are other start up delays (e.g. autocomplete, starting the kernel). I've never known starting the kernel to fail. I'm not sure on autocomplete.
Once start up delays are resolved, performance is good.