JS is slow, but I program Rust and Scala. Every internal app at my company is react based and I spend most my time waiting for that bloated framework to load a simple table. It can take seconds to load a 20 route paginated table from source.

JS is not slow.

Since JS is single threaded it can be pretty slow compared to anything being able to use multiple threads

JavaScript is slow if you need to do things that JavaScript can’t do, such as

• lots of stack-allocated objects/arrays (in JavaScript you have to heap-allocate them)
• hash maps with non-primitive types as keys (in JavaScript you have to serialize them to a string)
• count trailing zero bits (in JavaScript you have to use a lookup table)