You asked clearly for the runtime of the code. Distributing a variable by evalin() is a drawback, as explained by others already. It is faster to design a program such, that all used variables are provided by input and output arguments. This is the reason, why e.g. almost all functions of Matlab's toolboxed use this method. (Exceptions: disp and save, which are driven by using the names of the variables instead of their values.)
Stephen Cobeldick hit an important point in his comment: evalin() makes the code hard to debug and to maintain. If your code is useful, you will use it in other applications. Over the time such codes will grow. In my case some tiny scripts with 20 lines of code, which fixed some inputs taken from Excel files, grow to 227'000 lines of Matlab code since 1999. Now imagine that some of the functions use variables stored in the base workspace called "A1", and the GUI of this propgram is open, while some users run scripts in the command window, which use "A1" as name of a variable by accident. This will cause unexpected outputs and it nearly impossible to debug this.
Storing variables in the base workspace and accessing them by evalin is a source of serious troubles. The debugging may need an hour, or in my case some weeks, because I would have to visit the lab, which uses my software and they will not be able to reproduce the problem until the other user runs its script again. Even if the runtime with evalin() would be some microseconds faster than with other methods, an instable code design can slow down the time to get the results dramatically.
Remember: We write code to solve problems. The time, until the problem is solved, is ths sum:
t_solved = t_designing + d_programming + t_documenting + ...
t_debugging + t_optimizing + t_runtime
Trying to improve the processing speed, while the code is still fragile dur to evalin's is called "premature optimizing" and a common reason for instable and in consequence not usable code.
Your question shows, that it is time to enter the next level of programming by learing, how to design code efficiently. Software engineering is important and more or less unknown for too many scientists. I've seen a lot of programs developped for e.g. a PhD, which are an undocumented pile of scripts which must be called in a specific order to obtain some results. As soon as the PhD is finished and the author lost the post-it on which the needed order was written, this software is useless and worth to be deleted soon.
Do not try to improve the runtime, but spend your time to refactor the complete code, such that all variables are provided either by inputs and outputs, of by sharing the variables using inlined functions. Take into account, that good code will be reused in other projects, and this is only possible, if the functions have a well designed and documented interface. evalin() is a secure indicator for messy code, so find a way to avoid it.