I would respectfully disagree with Ron (RoHe): If your system isn't actually starved for memory, adding more won't make it run faster, or feel faster. I'm posting this from a (non-Dell) laptop with 8GB, and it runs just fine. Actually an important question I forgot to ask: what sort of work are your parents doing on the AIO? I guess I was assuming the usual web browsing, YouTube, etc. but I shouldn't have assumed. Maybe they're editing video and playing Call of Duty. I would say that if—BIG if—everything is operating normally, having an SSD as your boot drive would make the system feel much faster, especially booting and launching apps. (You could add a small-ish M.2 SSD to boot from, and still keep files on the HDD. It'd require reinstalling Windows though.) But if you weren't exaggerating about delay times on the order of a minute or more, there is definitely something very wrong. If the problem is the one I described earlier, doubling the memory won't help much: it'll just take a little longer to fail. If Task Manager shows that Desktop Window Manager is blowing up, kindly check what graphics you have. I believe the i3 version of the 5400 only comes with Intel 630 graphics, but to be sure you can run "sysinfo" (from Windows search) and check the name provided under Components / Display.
I bought my parents a brand new Inspiron 5400 AIO for Christmas was a Black Friday sale for like $550. I set it up in January for them and it is so slow its barely able to be used. If you try and type it takes at least 1 minute for letters to show up. I called the support and they said it was a software issue and I had to pay $99 to talk to support for that my warranty only covers hardware.    How does Dell charge over $500 for a computer that does not work, i also tried to return and they said it was past their 30 day refund policy. What a scam this desktop is garbage and Dell wants more $$$ to even talk to somebody about why. 
Inspiron 5400 AIO so slow brand new unusable