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)

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.