They likely don’t scale to specific Seekers - but may vary in speed.
It seems unlikely that the Snitch adjusts its own difficulty to the specific two opposing Seekers playing the game - otherwise there wouldn’t be any matches that drag on because the Seekers can’t catch the Snitch, and Harry wouldn’t be able to catch the Snitch in the first five minutes because the Snitch would’ve made it harder for him.
It’s possible that there are Snitches made with “innate” levels of difficulty, that wouldn’t scale to specific Seekers, but would naturally be a certain difficulty to catch. For example, there might be a Hogwarts-level Snitch, a professional-league-level Snitch, and a Quidditch-World-Cup-level Snitch, since it mightn’t even be possible for a Seeker at the lower levels to catch a Snitch intended to evade capture by world-class players.
Snitches don’t always fly at the same speed, one Ginny caught ‘wasn’t very fast’.
Snitches definitely aren’t all the same speed. That much, we can see. Ginny mentioned that the Snitch she caught during a game at Hogwarts “wasn’t a very fast one”, which shows that all Snitches don’t always have to fly at the same speed. That Snitch was still considered usable, and Ginny didn’t seem to find it particularly odd that it was a bit slow, so it’s unlikely Snitches have to fly at a standardized speed.
“Good catch,’ Harry told Ginny back in the common room, where the atmosphere resembled that of a particularly dismal funeral.
‘I was lucky,’ she shrugged. ‘It wasn’t a very fast Snitch and Summerby’s got a cold, he sneezed and closed his eyes at exactly the wrong moment. Anyway, once you’re back on the team –”
- Harry Potter and the Order of the Phoenix, Chapter 26 (Seen and Unforeseen)
She seems nonchalant about the Snitch not being very fast, not surprised like would be expected if this was a particularly unusual occurrence. Instead, she considered it fortunate but not shocking.