I don't know the details in this case, but very often, any use of eval() prevents a lot of optimizations. That's because for anything the eval() can see, all static analysis goes out the window. In a dynamic language like javascript, there may have been precious little of it in the first place, but a lot of smart people have figured out a way to scrape together some run-time optimizations based on them. The existence of eval() tends to kill them.
The parent isn't talking about runtime-eval (where you hit the interpreter with strings over and over in a hot loop), but rather "manual JITing", the technique where you take code that would otherwise be very dynamic (looking up method names using variables, etc.), generate a string containing a function definition of one particular concrete specialization of said code, and then eval that string to get a native function handle—just as if said code had been turned into a blob URI and shoved into a <script src=""> attribute. Those functions will then get statically analyzed just like any other functions.
