I don't think PHP/MySQL is the issue. I run a site with a single 3TB MySQL database and have no problem pulling and charting the data in fractions of a second. Some of the tables are in the area of 10 billion rows.

For comparison, I've also worked with a large variety of dashboards from companies using Snowflake/Tableau that are paying teams millions of dollars a year to build their interfaces, and it's normal to see sluggish load times that are taking 3-10s+.

The big difference is I know what charts and data I want to display, and I build my database from the ground up so it's optimized to show that data. These companies have teams of people that know absolutely nothing about databases and data structures, they just drag a bunch of data together to create some temporary table, then pass that data over to another piece of software. That gets imported into something else where someone that can barely write SQL writes a poorly optimized statement that somehow shows the expected result. They have more flexibility, and everything loads, although performance isn't ideal. Mine either works in 0.1s or it'll never load.