Sometimes. The phrase "things are only as slow as the slowest link in the chain" comes to mind.
As someone who owned and used a 750GB 7200RPM SSHD, I can share from personal experience that these types of drives perform very well in most situations, very SSD like most of the time.
An SSHD is a hybrid between an HDD and SSD
Are you kidding? Did you honestly think you needed to say that? Come on now.
and unless all of the Windows and application data fetched during boot is on the SSD portion (and this is presuming the SSHD can actually "store" it there as opposed to using the NAND portion entirely as cache), then it might not have (much) faster performance here. The thread starter mentioned having a 1 TB SSHD, and I recall that most SSHDs of around that size had pretty small NAND sizes of like 8 GB. I don't know what exact model it is so I can't say.
You clearly don't understand how they actually work. Try to remember, 8GB is enough space to store an ENTIRE Windows 11 install, WITH drivers. 8GB is more than enough to cache the most frequently used sectors of a drive, which is what they do. So unless they(the OP) NEEDS the entire drive to perform like an SSD, their SSHD will continue to do just fine as is.
In any case, I don't see any indication of "I'm using all of my RAM" so my first guess is that bumping 16 GB to 32 GB won't help boot times (or anything), but the "slow boot" mentioned would have me looking at either the SSHD, an overloaded startup Windows install, and/or just the older 12 year old platform (since they mention the Coffee Lake platform is slightly faster and also uses an SSHD). I had an older Core 2 PC and moving from an HDD to an SSD was an unbelievable improvement in startup and overall responsiveness even on that. Even if the Haswell system is a dozen years old, it's still fast enough that saddling it with an HDD as the boot drive will vastly slow its startup and general responsiveness.
Again, spoken like someone with no real-world experience with an SSHD. Here's an idea, get on Ebay and find yourself a 750GB or 1TB SSHD, buy it and try it out. Then come back and lecture us again on how well they work.