One comment I'd like to make here too, is a
rule of thumb guide many of us "old guys" use. If the computer your grandson is using (it would be helpful to know the make/model) doesn't boot up in less than 5 min. it needs to be serviced. You can do a lot of this stuff yourself as mentioned by others above, however, this is tricky stuff. To be clear, from the time you turn on the power-on switch on your grandson's computer until the time Windows boots up and you can bring up a browser (IE, Chrome, Firefox, Safari, etc.) to a home page; this is bootup time, and on a PC that came with Win8.1 it
should not exceed 5 min. total time. Otherwise it's a combination of Windows corruption, possible virus/malware infection, or faulty hardware, which takes a lot of work to narrow down and fix.
Tweaking the startup and using
MSCONFIG are only troubleshooting tools, and are usually meant for Techs to use on a
temporary basis. Many of my Clients attempt to make their computers run better by "tweaking" the
MSCONFIG and leaving it run in
DIAGNOSTIC or
Test mode, and never restore the
MSCONFIG to normal mode

. Not doing this means that the real problem, software or hardware caused, has not been found and repaired, so the computer is never really running normally; even though it appears to run faster. If the PC came with Win8.1 it's probably 4 to 5 years old and never had a professional servicing; hard drives and RAM often fail during that period, so your hardware needs to be tested to eliminate that from the mix. Also, online Gaming serious degrades the PC's effective life and the "rule of thumb" bootup time test will tell you if that's causing problems.
On the other hand, if that computer is older than 5 years old (>2011), and it was upgraded from an earlier version of Windows (XP, Vista, Win7) to 8.1; it's quite likely that old hardware has begun to fail and needs replacing, as well as drivers and BIOS are likely both out of date.

Again, professional servicing may be able to identify if this is the case and get all the updates in and replace any faulty hardware found. If you have a newer computer, say 3 years old with Win8/8.1/8.1.1/10 on it, take it to his place and load up his game and test it. If it's fine on your test PC, it's very likely one or more of these aging type problems. 9/10 computers >5 years old need their hard drives replaced, so most likely if your Grandson's computer was upgraded from 8.1 from an earlier version of Windows as mentioned, that's the first thing to try.
Let us know how it turns out.
Best,
<<<BIGBEARJEDI>>>