[flukus]

I'd bet the winforms version would be faster without DevExpress. We use the web version at work and it's extremely apparent that their use cases are small shops without a lot of data, they do a lot of things in incredibly inefficient ways that require/encourage encourage n+1 problems and things like server side (database) paging are like pulling teeth. To get a page working with any sort of efficiency you have to fight the framework which takes more time and effort than you save from using it.

For the JS stuff, a lot will depends on how you add the data because things like DOM redraws can be brutal. If you have a thousand items in a list for instance, adding them to the list 1 at a time could trigger the DOM to do whatever it does a thousand times, but if you can add them in one go then the DOM only changes once (with a decent framework). Whatever you're using you have to know how it interacts with the DOM.

I'm not sure how ag-grid works, but I'd be willing to bet that any table built with a DOM like structure will always be orders of magnitude slower (and eat more ram) than a real grid that simply paints.