As soon as you hit a certain level of complexity in your solution/software/whatever, you will be using multiple languages. You'll have scripts written in Bash,

As soon as you hit a certain level of complexity in your solution/software/whatever, you will be using multiple languages. You'll have scripts written in Bash, front-ends using PHP, back-ends using C or whatever. You are definitely correct that there is no black and white here, all software just needs to be "good enough" to actually solve a problem and be worth paying for. If PHP being slow stops you from being "good enough" then it's very likely that you can just isolate the part where PHP isn't doing a good job, re-write that in another language and keep the parts where PHP is working fine.