Trading a VIX futures spread, short M1-long M2, and my margin is about half of the long position, or $13K. Isn't this high for SPAn margin? Or is it portfolio/Reg T because of the VIX's unusual index characteristics and it's a CBOE, rather than CME, product, if that means anything?
CFE Margin requirements can be found here. https://cdn.cboe.com/resources/margin_updates/2022/CFEMargins20221026.pdf. Nov is currently $11,000 while Month 1 vs Month 2 looks like $6545. Your broker can always charge more.
What specific months are you currently short and long? Here are example margins ( long nov/short dec):
Like @Robert Morse said, your broker may charge higher margins for different reasons. Hope this helps and trade goes your way!