I tried to load the CSV that I most recently imported into Google Sheets into this hosted Grist and it choked. The behavior of the progress bar implies some kind of super-linear algorithm.
The most important component of "alternative to Google Sheets" is function. "Open core" is an aesthetic matter of little interest to most users.
It's a Node app, so CPU-bound work will run linearly on a single thread unless work was done to break up work into multiple processes or worker threads.
The most important component of "alternative to Google Sheets" is function. "Open core" is an aesthetic matter of little interest to most users.