Fast is a relative thing. Clearly a properly handled, well designed modern boats will be faster on all points of sail and in all conditions.
But once you get past that kind of comparison, the Marlin was actually a pretty fast boat for its era and intent. The hull design is such that for a long keeled boat it has comparatively little wetted surface. As with many Herreschoff designs, its waterline was long for that era, its bow was comparatively fine, and the run and counter was straight, clean and close enough to the water to be helpful. These boats had a pretty good sized sail plan, and the original fractional rig
was an easy rig
All of which is to say, these boats sail well and were comparatively fast for their day. And that performance can be improved over the boat in stock form and as you typically find them, by making sure that you have decent modern shaped sails, an efficient mainsheet and traveler, and a vang.
If properly set up, you would expect to be able to reliably average 2-3 knots in a moderate breeze, and perhaps 4 knots in a really solid wind. That may sound slow, but average speeds are deceptive since you either need to do stay close to the average speed reliably and consistently, or be able to sustain higher speeds for short bursts to offset the slower speeds in the averaging.
The one thing that I will say about boats like these are that they have comparatively small carrying capacities relative to their displacement. When yacht designers evaluate how much a boat can carry, they look at the displacement of the boat and the percent of its displacement it can carry before it becomes dramatically slower and more dangerous. The good news is that these boats have a pretty high displacement for their length. But the bad news is that the perentage of displacement that a boat can safely carry is mostly governed by the area of a boat's waterline plane. Boats like these have comparatively small waterline planes and so can tolerate a smaller percentage of their displacement as carrying capacity.
In other words, if you are going distancel cruising, you need to be disciplined about not overloading the boat with too much stuff. Years ago, the norm was to cruise very spartanly. But that expection has changed for most folks, and so there is an increased expectation about the sheer amount of stuff that people feel that they need to drag along. More and more, boats get loaded down with all kinds of gear either for 'safety' or in an attempt to approach all the comforts of home.
Neither approach is inherently right or wrong, but this type of boat does not permit the choice of cruising in a fashion which is 'just like home'.
Lastly, the Marlin was designed as a daysailer, overnighter, and when the cruiser version was built, it added a lot of weight which would further reduce excess carrying capacity and performance.
Great boats though.....