In a bit of pickle with the investors for a startup I work at – advice needed

I'm currently working at a startup along with two other cofounders, we're three years in, and a few employees. I find the two other co-founders completely incompetent (our head of marketing/cmo and our ceo). I'm the only woman and I feel like I'm being completely shut down by the CEO, a guy who not only happens to be incredibly intolerable to work with – nearly impossible – but also a massive narcissist. He's the typical know-all guy, cannot/won't accept any advice or criticism. He'll shut down whatever idea I might have, only to come around a few months later suggesting the exact same ideas I proposed and completely neglecting the fact that he got those ideas from me. When I call him out on it and tell him that I brought idea X up a few months ago, he sighs and tells me I focus too much on minor, irrelevant details which is incredibly infuriating. I try to be as professional as possible and brush off all the insults but it's reaching a point of no return.

I am by no means flawless, I know what I'm good at and what I'm not good at, but he's criticising me on my area of expertise and shutting my ideas down that I know could work for us. There's a reason I have my CXO role and he's the CEO - he's great with investors and the face outwards which is just what the investors happen to see, but internally he's a mess and completely incompetent from running our business.

We just received a round 3 funding from our investors and I sense that the investors aren't happy with our performance. Things are going okay, but they're obviously expecting more. Just this week one of investors asked me (and me alone) if we could grab lunch next week - I have a feeling they'll ask me about our lack of progress and the synergy with the other co-founders and I'm not sure how honest I should be. I want to tell them that our ceo is completely incompetent and that we're getting nowhere because he's a giant asshole who refuses to collaborate, but I also don't wanna throw him under the bus and completely ruin the ties with the investors because one day I want to jump ship and I wanna leave on a good note.

How honest should I be with the investor(s)?