For boot time and apps loading, all you need is just a SATA SSD, any brand, any model, the cheapest one and the most expensive one perform more or less the same. And all you need is just plug it into the optical bay SATA port.
If for any reason (obviously, other than boot time) you want to go PCIe SSD. Please make sure you get the PCIe AHCI SSD (if you may boot from it), NOT the NVMe SSD, and NOT the m.2 SATA SSD. Despite the form factor is m.2, a m.2 SATA SSD is still just a SATA SSD, won't magically match the real PCIe SSD's performance.
I have a $500 Samsung 1TB 840Evo on a PCIe SATA III card, and a super cheap $30 120GB DMG SSD on the optical SATA II port. They perform virtually the same for boot and apps loading.
I didn't own any real PCIe SSD, but since their 4k random read speed cannot even saturate a SATA II connection. I didn't expect there will be any noticeable difference on boot time or apps loading (unless there is a huge library associated with a particular app). Also, form the above graph, you can see that the connection type (or bandwidth) is practically irrelevant to the boot time and apps loading time. As long as you are using SSD, the loading performance are more or less the same.
So, if you have lots of money to burn, then go for a PCIe SSD, Kingston HyperX Predator 960GB AHCI is a good option, 100% OOTB solution. Samsung SM951 512GB AHCI is a good option as well, however, only OEM drive available, no warranty, and you need to buy an adaptor for it.
If you want something more cost effective. Then just get the cheapest SATA SSD that you can buy, and look for a size that you need plus at least 20% buffer. e.g. If you need 230GB for OS and Applications, than you should NOT go for the 256GB option, but 480GB option (or above).
Anyway, the post above mine show you the PCIe SSD SEQUENTIAL speed, which is totally irrelevant to the boot time, and very irrelevant to apps loading time. What you really need is the 4k random read speed. Which usually about 30MB/s for most SSD you can buy (including the SM951). And a normal HDD's 4k random read is about 1MB/s, that's why the boot / apps loading time is significantly faster on the SSD. IOPS does matter, but not the sequential read speed in this aspect.
I made this "apps loading" video some time ago. It's the 840 Evo plunged into the optical bay's native SATA II port. Obviously, a "6x faster" PCIe SSD won't really make the apps loading time 6x faster. Even yes, I don't think it's a matter to normal human being. It's still "very fast".