I am going to echo most, if not all, that GalaGirl has said.
I tend to use different words, and this may or may not help. I don't like the term "going at the speed of the slowest", because it implies that someone is "slow" which is negative, because they need to speed up in order to catch up. To me, that's too much pressure on someone that is perhaps already feeling guilty for "ruining people's fun".
Instead I prefer the term "going at the speed at which everyone is comfortable". This means going forward in a controlled way, communicating and doing frequent check-ins to make sure that nobody is getting steamrollered and feeling like things are careening out of control.
But, it's also important for the person who is not comfortable to be able to articulate why - to get to the bottom of what is causing the discomfort, so that the group can work together to find solutions. Don't make this a solo exercise, and the group shouldn't be labelling stuff as "your problem" - if the group is going to function then one person's problem is a *group problem* that needs to be solved. While this may well be one person having time to thing about it, the rest of the group can support that person by giving them the space that they need (without pushing), and being supportive in other ways. Sometimes the group synergy can be useful, because different people process things different ways - maybe by breaking it down as a group exercise, some idea can be found that the one who is not 100% comfortable would never have thought of.
I strongly recommend each individual coming up with a set of priorities for what they want - I usually use the categories "Need", "Want" and "Like" - comparing those lists can be extremely useful in terms of understand where different folks are coming from. It can certainly spark some good discussions.
"Listen, or your tongue will make you deaf." - Native American Proverb