PHP is super-fast today. I've built 2 customer facing web products with PHP which made each a million dollar business. And they were very fast!
https://dev.to/dehemi_fabio/why-php-is-still-worth-learning-...
https://dev.to/dehemi_fabio/why-php-is-still-worth-learning-...
I literally explained why this is not the case.
And Nextcloud being slow in general is not a new complaint from users.
On the other hand, in 99.99% of web applications you do not need self baked concurrency. Instead use a queue system which handles this. I've used this with 20 million background jobs per day without hassles, it scales very well horizontally und vertically.