MOVED: Inventory Freezes for several minutes when playing Multiplayer.

I’ll move this to Support & Bug Reports, since you report the problem occurs even in vanilla.

Question, Do you tend to have Creative Mode enabled all the time? When you have creative mode, you functionally have a second inventory that contains every item in the game, and having CM enabled will cause little lags like what you're talking about because it's loading every item in the game every time you open your inventory- On a worse off rig like your own, I could see that causing several seconds to as much as a minute.. On UL or other mods that add many more items, the problem would only get worse.

If you DO have CM enabled all the time- Try just leaving it off. Turn it on only when you need something from it.

"Cheat Mode" in the save options/server settings functionally just auto-enabled Creative Mode, cheat mode isn't required to be able to use creative though, or any other command.. So if you play in SP or MP and the 'cheat mode' setting is enabled, that's just enabling creative and causing this issue for you, in which case you'd need to disable cheat mode, or just use the CM command to disable creative mode.