They can only increase their block size (top 20 witnesses). but that is not good situation to deal with this issue.
Steem developers should feel and fix this problem.

Well, I've read the Whitepaper and increasing blocks is mentioned as a suggestion there. On the other hand, the Whitepaper says that Steem blockchain is designed to power a platform for more users than Reddit has nowadays.

What else may be done? Would better witness' servers help?

I understand.
That is not about witnesses. That is about steem blockchain.
They should change the way of configuring bandwidth between users in the blockchain.
Low bandwidth doesn't mean witness servers are not capable transactions. That means blockchain decided to allocate low bandwidth to the low SP users because high SP users are using blockchain and blockchain traffic is high. That's all.

Thank you for clarifying it. You have my witness vote.