Is it normal for a game with a lot of mods to take several seconds to open the menu when you either open a chest or go to buy something from a vendor? Sometimes it can take up to 10 seconds before the menu pops up.

Only if there is a massive amount of items in your/vendor's inventory otherwise no that isn't normal.

ah ok I'll try getting rid of some items then see if things work better. I have a carry all mod I use to help so I don't get over encumbered and get stuck in the middle of a dungeon.