-First, getting 4 sticks of ddr5 to boot and be stable can be a challenge in its own right. So getting 4x16 stable at 5600 on a z690 seems ok to me?
-Second, when you say recently added 2 sticks... this means that it's not a 4x16 kit. Even when you buy 2 identical kits (same make, model, etc), they're not guaranteed to work well together. Granted, you stand more chance of it working with identical kits than with totally different ones, but still.
If you need 64gb, you'd be better off selling this ram and getting a 2x32gb kit.
- Third, xmp itself is not a guarantee. So much depends on compatibility between the ram, the cpu and the motherboard. By that, I don't just mean the written specs, but what your particular silicon can handle and still maintain stability. As an example, I stupidly bought a 6400 2x16 kit for my 12700k, pro z690a wifi system.... tried many times to get it stable at that speed, but it always eventually results in instability of some kind. I thought I had it stable at 5800 before, but with the recent heat wave, even that is proving to have issues, so ive actually gone back to stock 4800 speed to maintain a stable and reliable system. To be fair, I don't notice any difference in games anyway. Point is, there's a lot of factors that affect xmp, it's not a guarantee, and will vary according to what your specific components can handle.
Is the system stable with just 1 kit, 2x16gb, and can you reach higher speeds and maintain system stability then? Do you actually need 64gb? If it's for gaming, you almost certainly do not, but if it's more for productivity or virtual machines/ servers etc, there may be a case for it.
With regards to the system taking longer to boot... is that just the first time, or every boot? First time you change ram settings in bios, memory training takes place and will take longer. I would also assume that, having 4 sticks, it would take slightly longer with every boot as the system still has to initiallize and check things each time.... but I don't know if this should take a LOT longer than with 2 sticks? Bear in mind that 4 sticks will always put more pressure on the cpu and system than 2. Some people even go as far as saying that all z690 boards should only come with 2 slots for this reason.
Anyway, maybe someone else can comment with full guides on how to stabilise and test the ram, but yeah, personally, I think 5600 for 5x16 is a decent result? Assuming it's stable.
