PoW makes it expensive to attempt a double spend. You can't practically double spend because it is too expensive to do. The high cost of PoW is what gives the network security from double spend. Therefore it is not wasted energy. It is energy spent to secure against double spends.
You could use PoS but that is centralized and vulnerable to manipulation just like fiat.
Slow depends on what you are needing the system to do. If you need to pay for groceries, then Bitcoin is slow. If you need to do remittances or settlement then Bitcoin is extremely fast - much faster to reach finality than anything else. Therefore Bitcoin is only really useful as a base layer of the financial world. But that is also extremely useful. Faster layers like Lightning can be built on top.
Bitcoin is a distributed consensus algorithm, and the way it establishes consensus is simple: pick the longest chain. Spamming dummy blocks on to an old chain will cause that consensus to switch, allowing double-spends, so Bitcoin requires PoW as an anti-spam mechanism to slow everything down by wasting energy ( https://en.wikipedia.org/wiki/Hashcash ).
I wouldn't go so far as calling blockchains "trustless", since PoW has its own vulnerabilities, e.g.
- Lottery-like incentives for mining favours centralisation, to reduce volatility. That gives mining pools a lot of control, e.g. whether to back forks or not.
- Centralised mining is vulnerable to existing power structures, e.g. those with deep pockets can purchase warehouse-sized mining rigs, can launch legal or physical attacks on the rigs of others, etc.