You're not going to get road bike performance from a mountain bike (weight, aerodynamics, gearing, energy lost in the suspension, etc). At least not dollar for dollar. But no doubt you would see a pretty good gain in performance going to a road friendly tire. As far as how much performance, too many variables to say. It's a reasonably cheap thing to try out. Pick up a set of tires and give it a whirl.
Let's say you put high pressure slicks on your mountain bike and on a twenty mile ride you average 15-16 mph. I would believe that on a reasonably equipped road bike you would increase your speed to 17-19 mph. I'm purely guessing. We have a guy in our club that can hang at 25 mph on his slick tired mountain bike in a group. On his road bike 28-30 mph rides are common. He is an exceptional athlete though.
I think the most important thing to obtain greater speed is your body postition. Road bikes aid you in fighting wind resistance better than a mountain bike. Yes you will go faster with wide slicks on a mountain bike ......just not road bike fast.
I get these types of questions all the time in my shop. It honestly depends on how serious you are about your riding. Mountain bikes are for riding mountains and trails, they are designed to best operate in a mountain/trail setting. Road bikes are made for road (paved) conditions, their purpose is to put down miles as quickly and efficiently as possible. If you're riding primarily road and wish to increase your time performing such activities, GET A ROAD BIKE. But if you still intend to ride offroad, a descent pair of road slicks for a 26" mountain bike will run you about $60-$80. Much cheaper than a $900 entry-level road bike. (for future reference if you decide to purchase a road bike: do not cheap out on it, you'll hate the experience and may spend hundreds and hundreds in upgrades to the bike. Expect to pay somewhere in the high $700's for a descent, entry-level road bike)
I could probably put out the arguement that a road bike is still not the most efficient way to put down miles under human power only, but this is the wrong forum for that discussion. To put out 75 to 200 watts less at any given speed you'll have to visit a different forum.
You will save a lot of money. Will you get the same performance.. obviously not.
There are few main disadvantages - tires, wheels, suspension (if you have), position on the bike, gearing.
I've done it once - I installed slick 26x1.50, rigid Kona fork, lovered and stretched the position. The bike was not heavy ~25 lbs and on the road I could make 18-19 mls for an hour but of course with much more effort compare to road bike... always on smallest cog.
I replaced it with the hybrid (w 700 wheels) and it was already an improvement... but still not the road bike.
Given that he's riding an Apollo, that tells me two things. First, he's probably in Australia, where bikes are not exactly inexpensive. Second, if he's got an Apollo, it may just be because he's not made of money. They're good bikes, and affordable. If his budget doesn't permit another bike, then the improvement he'll get with slicks would be worth it.
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