Check your graphics' control panel if the game's max frame rate is set to 60 or not.
Then make sure the background application max frame rate is disabled.
I had similar issue with the game
High Pressure in the past where for some reason, even though the game's window was active, it kept acting like it was being played in the background so the FPS was being capped to 30 the entire time, after disabling the said option, I no longer had such issue.
(Posting this for, anyone else who might have this issue, and replying cause, you did have the right idea, so cheers.)
I finally found what was causing this issue (stuck at 30 fps / half speed even on title screen), after I ran into another RPG MV game with the exact same problem and dug into it again.
I had previously been using AMD's graphics utility (CCC in this case) to correct an overscan issue I was having (video feed being displayed past the screen borders in preferred resolution), and apparently this is, somehow, causing the problem. (Of all the games I've played since, zero issues ever, only these 2 RPGMV games seem to take issue.)
Might also be something about using a "TV" as a monitor, which was something I'd run across when searching this issue again, I don't have a regular monitor at this time to confirm if this is solely caused by the overscan adjust, or a combination of the two.
Temp fix is to un-adjust/re-adjust this setting as needed, but a better solution might be to run these games in fullscreen (that's "dedicated" fullscreen, not the windowed fullscreen you get from F4). I'm, unsure how to force this, I tried a plugin I'd found, but it only forces windowed fullscreen on start. Previous RPGM iterations had a config file alongside the exe you could change to do this, but I guess that's been phased out or something, so, unsure how to do this for RPGMV.