Yes, it's significant. Unfortunately, there are fundamental trade-offs here between protection and bandwidth and/or latency. Another aspect is energy: keeping a connection "alive" by regularly ensuring traffic on a connection does not help battery life. We have much to optimize here.
It's similar to how encryption was viewed as too expensive a decade or two ago. Today, it is a necessity. Seeing how available bandwidth keeps growing to accommodate things like video, I hope traffic analysis defenses won't be as detrimental in the long run for most internet use.
Yes, for sure. As a defender, you have two main tools: dummy packets (bandwidth) and delaying packets (latency). Padding-only defenses will indirectly delay normal (non-padding) packets by filling the connection with padding. You want to explicitly block outgoing traffic and try to account for congestion to minimize wasted bandwidth.
This is tricky. We have hardly started dealing with traffic analysis issues in protocols. In general, we have spent the last decade+ getting encryption sort of right with amazing efforts like TLS 1.3 and WireGuard, etc. Expect another decade for traffic analysis.
(Disclosure: I work with Mullvad on DAITA.)