We'll find out soon. Before that, some clues.
On apple.com website it says:
"Metal speeds system-level graphics rendering by up to 50 percent (Footnote: Testing conducted by Apple in May 2015 using 2.7GHz Intel Core i5-based 13-inch MacBook Pro systems with 128GB of flash storage and 8GB of RAM. Tested with prerelease OS X v10.11. Not all features are available on all devices. Performance will vary based on system configuration, application workload, and other factors.)
...as well as making it up to 40 percent more efficient. (Footnote: Testing conducted by Apple in May 2015 using 2.2GHz Intel Core i7-based 15-inch MacBook Pro systems with 256GB of flash storage and 16GB of RAM. Tested with prerelease OS X v10.11. Not all features are available on all devices. Performance will vary based on system configuration, application workload, and other factors.)
...Metal allows the main processor and graphics processor to work more effectively together, boosting high-performance apps. And Metal is designed to be great for games, improving draw call performance by up to 10x and paving the way for new levels of realism and detail. (Footnote: Testing conducted by Apple in May 2015 using 2.7GHz Intel Core i5-based 13-inch MacBook Pro systems with 128GB of flash storage and 8GB of RAM. Tested with prerelease OS X v10.11. Not all features are available on all devices. Performance will vary based on system configuration, application workload, and other factors.)
...UP TO 4x FASTER PDF OPENING IN PREVIEW (Footnote: Testing conducted by Apple in May 2015 using 2.7GHz Intel Core i5-based 13-inch MacBook Pro systems with 128GB of flash storage and 8GB of RAM. Tested with prerelease OS X v10.11 and shipping OS X v10.10.3. Not all features are available on all devices. Performance will vary based on system configuration, application workload, and other factors.)"