Yeah, your right: https://aws.amazon.com/lightsail/faq/
I haven't really touched Lightsail in years, but this page shows a really nice graph that helps you understand your instance's CPU usage: https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amaz...
From what I've seen of benchmarks online, DO's droplets have better sustained (and often overall) performance than Lightsail, and this looks to be the reason why.
(Which isn't to say people should avoid Lightsail, but I don't think Lightsail is the obvious 1:1 replacement for DO that a couple of people in this thread want it to be. Lightsail intentionally restricts various things to avoid cannibalizing their normal AWS sales.)
The last time I checked Lightsail uses similar CPU credits[0] as their t2 ec2 instances.
As long as you're only using a tiny portion of your CPU it's fine but if you start doing work on your instance where your CPU is being used for a sustained amount of time then you run out of CPU credits and performance is drastically degraded.
DigitalOcean has no such mechanism. I've never had a droplet's CPU performance get penalized because I used the allocated resources that I was paying for.
[0]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstabl...