Yes, thinking between plays is totally normal. 10 seconds is more than acceptable, rather lower I'd say. Slowplaying usually means taking 3 min to play a Copper , then 3 min to play the next Copper, etc.
In games some players always play faster than others, that isn't unique to Dominion. The fact that online Dominion allows fast setup, no time required for shuffling cards etc. makes some players think turns should take 5-10s for everybody because they do it that way.
Remember you have 2 options. You can close the chat window by clicking on the "speech bubble" icon. You no longer need to see what they write. If you think you'll not want to play that player again, you can blacklist them right during the game.
Click the icon with the 3 "Meeples" , then there should be a heart icon (friend) and lightning bolt icon (blacklist) for each non-bot player and spectator. Click the lightning bolt icon and (if it's a spectator) they are kicked from the table and if it's a player, they are muted for you.