That's a speed issue, not a menu position issue.
Menu Bar Placement Poll

Also, they're slow. Takes a split second to see the dropdown.
A whole "split second"?

While I used to use top, I understand the advantages to side. For one, it's always there as you scroll up/down the page; they could do a floating menu bar at the top, but that takes more code and more maintenance work than a simple fixed sidebar.
Also, that's the way web design is generally going, so going forward that's what's going to be more intuitive to new members.
I’m not that much into web design, but when I made a website last time, floating top menu wasn’t that difficult. One line of css for a simple static menu, a slightly bit more for the “roll out” menu, or how is it called. Should be a piece of cake for a professional web designer.
Also, I’ve seen some freshly looking sites of a great design with a top menu bar. I’d say some web design goes for the top, some prefers to go aside.

One advantage to the side is that it doesn't take valuable vertical screen space. On my laptop, I have tons of spare horizontal space, but anything vertical means I will see less of the forum and need to scroll a little more.

In general, it’s a bad idea to offer a functionality to users and then take it from them. I wasn’t using the top menu, but I think it wasn’t a good decision to take it away.

Also, they're slow. Takes a split second to see the dropdown.
A whole "split second"?
Yes, any delay can be annoying.

I get it, it’s better on the side than on top for most people. But having a choice, isn’t that even better?

I get it, it’s better on the side than on top for most people. But having a choice, isn’t that even better?
Yes, it really is. Satisfies both parties. Sorry if Chess.com thinks it's too hard or whatever, they'll find a way to make it work, if they're being paid.

Also, they're slow. Takes a split second to see the dropdown.
A whole "split second"?
Yes, any delay can be annoying.
Were you looking for something that would take you back in time? How fast were you looking?

Also, they're slow. Takes a split second to see the dropdown.
A whole "split second"?
Yes, any delay can be annoying.
Were you looking for something that would take you back in time? How fast were you looking?
As fast as the top menu. Immediate response.

Also, they're slow. Takes a split second to see the dropdown.
A whole "split second"?
Yes, any delay can be annoying.
Were you looking for something that would take you back in time? How fast were you looking?
As fast as the top menu. Immediate response.
How has this drastic wait in time impacted your life?

Also, they're slow. Takes a split second to see the dropdown.
A whole "split second"?
Yes, any delay can be annoying.
Were you looking for something that would take you back in time? How fast were you looking?
As fast as the top menu. Immediate response.
How has this drastic wait in time impacted your life?
It hasn't. Not sure what you're trying to get at here.

One problem with these kinds of online polls is that the people who are happy with the current situation are less likely to be talking about the subject in the forums and therefore less likely to bother voting - and less likely to even know about a poll.
I also suspect that if you were to repeat the poll in six months once people are familiar with the new layout, you would get a different set of answers.
Also, they're slow. Takes a split second to see the dropdown.