As Brian said, the throttling is done by the ISP, which a VPN has no control over since it still has to connect from point A to B.
In my experience, the main things VPNs can affect are 1) download speed, 2) how well some web sites work, 3) ads, and 4) logins.
If you manage to get a decent VPN though, only website functionality and login are a problem, and only on a small percentage of sites. If you use streaming services though, it can be annoying when they don't work. I use PlutoTV a fair bit, and even though it's a free service, I have to use it without my VPN running.
Very little risk of malware or viruses, and most of the better ones block ads pretty well too. In fact the main difference you immediately see is all the ads popping up without it running.
I mean I still run malware scans every now and then, but since using a VPN, they never catch anything. Dynamically masking your IP address is a pretty foolproof and easy way of blocking shenanigans of all types, including keyloggers.