Which is faster, Executing a program from ROM or RAM?
The datasheet should tell you how long each instruction takes, and what differences there are, if any, between executing from RAM or ROM.

For microcontroller that offer the option of executing from RAM, that is probably faster, likely being the main point of using additional RAM space to execute code from.  There may also be some fetch overlap issues.  In some cases it might be faster to execute from ROM because it is a separate memory and RAM access can be going on concurrently.

Again, the only way to know for any particular micro is to READ THE DATASHEET.