This is bad news for Slack, I would think. I believe they went all in on HHVM + Hack, which is going to be increasingly incompatible with 3rd party packages now.
Is it possible they could find inefficiencies in their codebase to make up the performance difference with regular PHP? Or maybe they'll just need to add more servers to make up the difference?
Adding more servers stops being a good option after a certain point. If you're a small company running on 20 servers you can increase your performance by 5% if you add just one more. Install it in one day, boom.
If you're running on 200,000 servers, a 5% increase is 10,000 servers. That means opening a new building in your data-center, making sure your recruiting team has resources to hire enough staff for it, then hiring that staff, negotiating contracts with the power company, the component suppliers, building permits, hopefully you have enough network capacity otherwise you'll have to expand that too, etc.
It's many months of work and a lot more expensive than hiring a handful of engineers to tackle performance and maybe build some libraries in-house.
I think it depends on whether the improvements facebook hopes to achieve by dropping support for php outweigh the drawbacks of losing 3rd party package compatibility
