That’s… a rather huge drawback. Why even pay for a shield at that point?

And is hilariously overkill for what OP seems to want. It’s a pretty large and heavy package that comes with a whole lot of (for OP unnecessary) features.

Pi-hole’s not a router, just a fancy DNS server. Your network traffic doesn’t go through it, so its impact on your speeds is negligible. Since all it does is respond to DNS queries and keep logs, it also doesn’t require a lot of processing power. I used to run it on the first gen raspberry pi, and even that puny thing could easily handle the job. Your Synology box should be able to do it just fine.

Get one of them mini PCs that they attach to the backs of monitors at office desks or receptions or whatever. Something like a Lenovo m720q for reference, though there many other similar products from other companies. They can be had for pretty cheap on the used market where they are abundantly available, they’re very power efficient (obviously not as efficient as a pi but still pretty damn good), and they’re surprisingly powerful for how small they are. I’d actually recommend a machine like that over a raspberry pi. Pis are great when you want the smallest and most low power machine you can get, but at the end of the day it’s an ARM based machine with very limited IO. A regular ol’ x64 machine with bog standard sata and m.2 ports all inside a neat enclosure is also great.