That's incorrect, while GTASA is ancient, it is pretty well optimized on its own. Look at GTA IV for example, which is complete garbage when it comes to optimization.
The issues primarily originate from the SAMP and LSRP scripts not being fully optimized. As is, SAMP is very poorly optimized, especially in the synchronization department. However, it's not just SAMP, some of the irregularities originate from the LSRP scripts, which suggests that they also lack optimization. Reviewing the performance profiling made by Mmartin suggests that there are several curlpits being the garage system and several native functions.