It really depends on what you're doing. If 95% of your code is file or network I/O then it's really not going to make the slightest difference whether you're running JSC, V8 or QuickJS. If your code is enormously complex and benefits from JIT then yes, you're going to really feel that downgrade.
The difference in real world performance between web servers written in things like Python or Ruby to those written in Go, C# or even lower-level languages like Rust would indicate that's an over-estimation of how much IO dominates total runtime, even if it is by far the slowest part.