this is the design of all portable Macs for the last decade or so.
Try closer to 2 decades.
A couple of years ago, I sold a friend a spare 1ghz Titanium PowerBook from my collection. I sold it to him with full disclosure of the fact that the battery was completely shot in it, but I hadn't really used it other than powering it on and installing an OS.
He started using it, and claimed that it just didn't "feel" as fast he expected, and we compared notes on how fast certain specific things were on the 1ghz I kept.
He dug into some more intensive benchmarks, and then finally found a few things backed up by contemporary Apple support documents. At that point, the battery was completely dead-neither OS 9 or OS X would even see it, so even installed it was effectively like not having one. The system would automatically down-clock to 667mhz and disable the L3 cache.
With a good battery, the performance returned to what was expected.
On Intel systems where the battery can easily be removed, you can see/feel the performance hit to the point where they can become nearly unusable without one. Even at that, I picked up a 2011 17" not too long ago(I know, a ticking time bomb, but I'll enjoy it while it still works and then disable the dGPU) and it arrived with a swollen and dead battery. The initial set-up was miserable, but with a new battery(and I bought a good one from Newertech) it is everything I expected.
On the other hand, I have a 2010 15" MBP where the battery will charge up and register 100% with perfect health, but it's "hiding" that the battery is actually near dead. When unplugged, the computer works for about 5 minutes without registering in drop in charge status then suddenly shuts off. I did a small experiment the past week where I powered up a program that can peg the CPU and that I've seen drain the battery even when plugged in-Folding@Home. Plugged in, it will run 20-30 minutes before the computer will just shut off.