That feedrate slider is probably the feedrate adjustment youre looking for. Octoprint has one too. It translates into M220 I think. Octoprint also has pause/resume function and you can add extra scripts, for example if you want to stop the spindle on pause and start (and dwell a bit) on resume.
The biggest issue like Jeff said is it doesn’t take effect immediately. There is a movement queue of about 16 segments that must be depleted before the feedrate change or pause takes effect. Depending on the job, this could be a long time if there are many long straight segments. Surfacing a spoilboard this can easily be five minutes, which makes it worse than useless.
Also an instant pause seemed particularly useful but it doesn’t exist in Marlin. I implemented a function for an external pause button which was pretty simple (but doesn’t respect acceleration, so it carries some risk). It was not accepted into upstream Marlin but you could aplly the patch yourself.