This is according to reliable tipster Mark Gurman and his "Power On" newsletter (opens in new tab), which claims that the Series 8 S8 processor won't be any faster than the S7 chip in the Series 7.
The S7 chip has the same specification as the S6 chip that came before it.
Now, that's not necessarily an issue, as the Series 7 is no slouch, but if you're hoping for a big upgrade then it sounds like you might be in for a disappointment.
Later in the article, Gurman does state that he believes the Apple Watch Series 8 will feature a low-power mode which extends the battery life. He believes this could be enabled with hardware, which does suggest either a low-power co-processor or a reworking of the current S7 chip.
If you're waiting for a faster and more powerful Apple Watch, then you'll have to wait until the Apple Watch Series 9 in 2023, as this will reportedly get an all-new processor.
Back to this year though… Alongside the Apple Watch Series 8, it's expected that Apple will introduce an updated Apple Watch SE and a new rugged Apple Watch model.
All of these models could use the same S8 chip.