You really need to play the speeds to see the difference.
Whenthe game lasts 3 times as long and things take 3 times as long to build is it much difference? The real differences lay in things like units. Once built a unit still moves normally and fights normally. If you lose units to an attacking force it is very hard to get those back fast enough. If that makes sense. On online speed you just punch out the units. Now that may sound easier and in some ways it feels like it but the less turns and speed change does mean you miss some little benefits. It is very easy on online speed to suddenly wonder where all the turns have gone and you still only have 3 cities.
I get that. I'm just wondering if the early game is much harder on Marathon since the AI has three cities and lots of units, but you have the one city and one Warrior for quite a long time. Surviving early game rushes from the AI might be tough. I suppose that, eventually, you catch up and then the slow speed works in your favor.