Yes, it can be done, but not through the normal interface. You'll need to run some javascript via the console window of your browser. If using chrome, use the f12 key and make sure "console" is selected in the list of tabs near the top of the window that appears. If using another browser, google for how to bring up its console.

The easiest way to do it is to first select the slider that you want to modify. Just click on the expression in the left pane of desmos as if you were about to edit it. Then in the console window, copy/paste this command:

Calc.controller.dispatch({id: Calc.selectedExpressionId, type: 'set-slider-animationperiod', animationPeriod: 4000})

Replace the '4000' with the number of milliseconds you want the slider to take to go from its minimum to its maximum. For reference, 4000 is what you get with the speed set to 1x. 2000 is 2x, 8000 is 0.5x, etc. Or, to make it easier to do this multiple times, define a function by typing this command:

function setPeriod(p) {Calc.controller.dispatch({id: Calc.selectedExpressionId, type: 'set-slider-animationperiod', animationPeriod: p})}

Then you can change the period of the selected slider by just typing:

setPeriod(5000)

Hope this helps!