Initially I was using: PHPExcel (https://phpexcel.codeplex.com/) which loaded a given spreadsheet on demand - this was before I though of changing it into a client server model. In a web application context, the response time for anything other than relatively small spreadsheets was simply unacceptable.
I then moved to using Apache POI (https://poi.apache.org/) and used the client server model. This worked quite well but I ran into some problems as some functions had not been implemented yet.
My current thinking is that LibreOffice will continue to improve both in terms of function feature completeness with Microsoft Excel. It may also improve in terms of performance and memory usage.
Any solution that re-implements excel/calc functions, other than LO, most probably has a smaller user/developer base. In my mind, the smaller the projects user/developer base, the more likely active development could slow to a crawl or stop entirely. Not such a great solution for years down the line.