Both. It slows down traffic because packets have to be resent, and because of TCP (which assumes that if packets are lost, the connection must be congested, and therefore data should be sent slower).

If enough packets are resent the connection may not complete or it may timeout (if a packet needs to be resent multiple times, for ex).

I'm the founder of NuevoCloud, and this is one of the problems we are working on. Basically, we can route connections around congestion. So for example, if your website is in dallas and the visitor is in germany, but the normal route has an issue.. then we might reroute it germany > virginia > dallas until the issue is resolved. So your website still stays up.

High packet loss makes things with a real-time dependency like gaming, streaming, etc. near-impossible.