Here are some possible causes.
First, in this modern age, when you type into a browser while on certain websites, every keystroke is sent to the site. This is done so that the site can provide suggestions, e.g., the "autocomplete" that you now take for granted, and the suggestions that Google provides when you type into their search box.
Given this, the first possible problem is that your Internet connection is slow. This would explain why the problem is worse at some times than others. A slow Internet connection can be caused by having a lousy Internet Service Provider (ISP) but can also be caused by slow connections to your particular location. Technology managers know how to "ping" sites and look at the latency to that particular site in order to troubleshoot connection problems.
Another thing that can cause a slow connection is if you have some other connection, to another site, that is using a lot of bandwidth. So, if you are downloading a big file, watching a video, etc., typing may get less responsive.
Another possible problem is anti-virus software. Some anti-virus software can absolutely kill performance because it looks at everything sent to and from remote sites, and everything sent to and from your computer's disk drive, and if the anti-virus software gets overwhelmed (which it does), everything can slow to a crawl. I have over a dozen computers and don't use anti-virus software on any of them. I have been doing this since before the millennium. I wouldn't recommend it for everyone, but it works great for me, and I get amazing performance compared to computers that I work on when I help other people.
Of course this would not be specific to one given site, and you would typically have problems at all sites.
One final explanation is that there is some sort of script or code running when visiting this site, and your browser is having problems running this code. The usual solution to this is to make sure you are running the latest version of the browser, or to try a different browser. If the problem goes away, or gets better when you do one of these things, this will give you a good indication of where the problem lies.
I've never had a problem when visiting this site, although I have experienced problems, similar to those reported by the OP, at other sites. The problems turned out to be bad script code at the "bad" web site.
It sounds like quite a few people are having problems, so perhaps the webmaster needs to do a review of the code that is running while people type.
