I don't recall performance problems being the main concern for the last (final) Sim City, but I won't say you are incorrect. What I do remember is
1. shallow game play compared to its predecessors
2. demand for always online and proven lies about offline play not being possible because of the game architecture
3. invasive DRM, during a time when invasive DRM was on everyone's mind
4. launch issues which, combined with the always-online requirement, meant a solid "plop" of a release.
5. EA was already negatively viewed at the time by many PC gamers
It looks like the wikipedia article for the game mentions some of these, and other issues.
> SimCity's sixth major release was announced on March 5, 2012, for Windows and Mac OS X by Maxis at the "game changers" event.[31] Titled SimCity, it was a dramatic departure from previous SimCity games, featuring full 3D graphics, online multiplayer gameplay, the new Glassbox engine, as well as many other feature and gameplay changes. Director Ocean Quigley discussed issues that occurred during the development of the title, which stemmed from two conflicting visions coming from EA and Maxis. EA wanted to emphasize multiplayer, collaborative gameplay, with some of the simulation work conducted on remote servers, in part to combat piracy. In contrast, Maxis wanted to focus on graphical improvements with the new title. Quigley described the resultant title as a poor compromise between these two objectives- with only shallow multiplayer features, and a small city size limit- one quarter of the land area of previous titles in the franchise.[2][32]
> The game was released for Windows on March 5, 2013, and on Mac in August.[33][34][35] Medium would later refer to the release as "one of the most disastrous launches in history".[2] The game required a constant internet connection even during single-player activity, and server outages caused connection errors for many users. Multiplayer elements were "shallow at best", with departing players leaving abandoned cities behind in public regions. Users were unable to save their game- with the servers instead intended to handle this- and so when users were disconnected they would often lose hours of progress.[36] The game was also plagued by numerous bugs, which persisted long after launch.[37]
> The title was heavily criticized in user reviews, and developer plans for post-launch updates were scrapped.[2] EA announced that they would offer a free game from their library to all those who bought SimCity as compensation for the problems, and they concurred that the way the launch had been set up was "dumb".[38] As a result of this problem, Amazon temporarily stopped selling the game in the week after release.[39] The always-online requirement, even in single play, was highly criticised, particularly after gamers determined that the internet connection requirement could be easily removed.[40] An offline mode was subsequently made available by EA in March 2014, and a mobile port entitled SimCity: BuildIt was released later that year.[41][42][43]
> It has been suggested that the poor performance of SimCity was responsible for the 2015 closure of Maxis' Emeryville studios, and the end of the franchise.[44][45]