There will be some people (okay, I'll call them jerks
) who will be upset with you, and will likely make mean comments, if they perceive you as moving too slowly.
I expect that 98% of that will be eliminated if you--just when the game is beginning--make sure to immediately type into the chat section, "I am a beginning player. I will move as quickly as possible, but my first few moves may be slow, as I review these new cards." Or, "Three of these cards are new to me. I'll need a few minutes right now to review them; but after that, I can move quickly."
The possible bad reaction is because there are a very few players who take pleasure in slow-playing opponents (ie, moving very very slowly, intentionally), just to be mean. I think that just about everyone remembers what it was like when we ourselves first started and did not know all the cards, and therefore moved a bit more slowly. Your main responsibility is to write and tell people that you're a newbie, so that they do not confuse you with one of those awful intentional-slow-players. If you write nothing, then it's reasonable that you'll encounter hostility. If you do write, and explain in advance, and other players are STILL mean . . . well, then they are the jerk, not you!!! :-)
p.s. As a general rule: If, during any game, you need to take an unusually long period of time, it is only common courtesy to write something in chat. "Someone just knocked on my door. Will be 2 minutes." Or, "Complicated move. Will need a few minutes to think about this."
As long as you show your opponents simply respect, it's been my experience that the people on this site are incredibly kind and patient. When I was a beginner, I even had players offer suggestions on some of the more complicated card combinations, or explain why they did a particular move. I've tried to also do this, under the theory that (a) It makes for a more friendly game, (b) If it brings up your skill level, then it will make future matches more even, and therefore more fun for both of us, and (c) it's just a nice thing to do.
I hope your playing experiences here are as stress-free and as enjoyable as my own have been for me.