Can't really tell, many things can cause this such as server overload, plugins/mods/hacks making lots of requests/query's, etc...
check your server average in your admin panel as well see what numbers you're getting, if it's normal then your vB is Okay and the problem might be in your server or "the user" connection.
If it's too high then try to disable your Plugins/mods one by one and watch the average load if it's dropping down then you'll know which Plugin is causing this.
start with the most recent Plugins you installed.
Good Luck.
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