Prioritise a process that is (by default?) low priority

not exactly low priority, GUI objects run in their own thread and will ignore scheduler rate anyway.

but i would exclude the possibility that a numberbox is noticable slow or inaccurate because of non-.optimal patching. it might be something else which casues the trouble.

under windows it might be a background process which slows down you GUI.

In a similar situation I opted for JSUI. Depending on OS and Max version you may also want to try mgraphics/jitter.

https://cycling74.com/articles/event-priority-in-max-scheduler-vs-queue/

Thanks for the suggestions! I should have mentioned that I am running Max 7.3.5 on OSX 10.12.6. JSUI would be great, but I have to have this working in a couple of days for a performance, so don't have time to learn it :-) I might try with Jitter though...

It's not that difficult in JS. An example is included.
By the way, Max8 has improved graphics performance.

Save this as flash.js:

var black = [0., 0., 0., 1.];
var white = [1., 1., 1., 1.]

function loadbang() {
    reset();
}

function set() {
    draw(white);
    refresh();
}

function reset() {
    draw(black);
    refresh();
}

function draw(color)
{
    with (sketch) {
        glclearcolor(color);            
        glclear();            
    }
}

Save this patch in the same location:

My guess is that the problem lies in you drawing the Max GUI with very large areas (big int boxes, panels, etc).
Test your patch with a Jitter approach and see if the performance improves. I bet it does.
Drag the Jitter window to the 2nd monitor and press the Esc key to go fullscreen.

@PEDRO SANTOS Yes, you are right about that. I wouldn't have guessed that the *size* of the panels would make a difference, but it does! It's true that it was running SLIGHTLY more smoothly in Max 8, but still not so good as to be reliable. I redid it all with Jitter and now it is working like a charm.

@JVKR I see what you mean! On this occasion I used Jitter, but I think that I will use JSUI in future. Thank you very much for your help.

panel is old and/or does something "wrong".

try if [lcd] is faster than panel.

i use lcd to display time-critical stuff with up to 50 fps on 20 years old computers and it works fine.