I currently have a set of Specialized Espoir Sport tires on my Secteur which are 30mm, I enjoy the comfortable ride but with experience I'd like to upgrade to a faster tire. What kind of effect on speed and comfort would going from a 30mm tire to a 25mm tire have? My tires currently weight 440 gram each.

Going from 30mm to 25mm won't necessarily lower the rolling resistance. Going to a lighter tire in the same size with more supple sidewalls and without a puncture resistant layer might be an improvement while keeping or improving the comfort level. But the Specialized Espoir Sport isn't a bad tire to begin with.

How fast you going now?

If you are riding around at 12mph and expect to magically go 18 due to a tire change, that ain't gonna happen.

If you're going 18 and want to go 20, that might work.

I see no reason to ride on a 25mm tire unless it's the biggest tire your frame will allow. You will notice no speed change at all going down from the 30mm to the 25mm, but you will notice a change in ride quality, as you will have to run the 25s at least 20psi higher to accommodate the thinner tire. The only potential increase in speed (real or perceived) would be brought about by the reduction in rotating mass, not the width. Don't go to a thinner tire, go to a lighter one. 440g is fairly heavy, even for a 30mm. My 700x28 (measure 31.25mm mounted on the rim) Continental Gator Hardshells are by no means a light tire, but still come in at 330g apiece, about 4oz lighter per tire compared to the Specialized. And while I care very little about component weight, if I can save 4oz per tire with no caveats, no sense in not doing it. I would much rather sacrifice 1mph in favor of a bike that doesn't buzz and jitter over the crappy roads I ride on.

Switching to a more expensive tire with better rolling resistance would probably make a bigger difference than just tire size.

I'd recommend a Continental gp4000 in 28c:
https://amzn.com/B00G8QGVPS

I've read that they run small, so I think a 28c (their "largest" size). The gp4000 is as close to a race tire as you can get without losing puncture protection. (I mean a 25c would probably be fine to).

Ideal tire size for speed depends on your weight. Like a 250lb rider is going to be faster on a "bigger" 25c tire than a smaller 23c tire. Whereas a light 120lb rider would probably be the fastest on a 23c tire.

The Continental Grand Prix 4000 S II tun BIG (most conti tires run smaller than advertised)

I measured my 25c close to 28mm wide
and 28c close to 30mm, excellent tires and about 270g each, much lighter than other 28s and 32s

28mm??? 30mm??? Ya might as well be riding a fat tire bike!!!

But srsly...when I started road biking 9 years ago I felt like a rebel going to 25mm tires...now it's the norm in my club.

I would say if going 18 and want to go 18.25 that it might work.

However a high quality 30 mm could even roll faster than a poor quality 25.

28 years ago my road bike had 19mm wide tires. Now I have a 25mm on the front and a 28mm on the rear (fork won't take any larger). I am going slower now, but certain that it is not due to the tires!

I flip between two extremes on the same bike. Most of the time I run 42mm Conti Crossrides, (420 g) but once a year I run an Attack /Force set 24mm rear 22mm front ( 195-175 g) for a month or two.

The only difference is PERCEIVED acceleration and of course the weight, comfort and the ability to go about anywhere without a second thought on the 42's.

There's no real difference in speed on flat pavement!

On the other hand I have run some 32's that were really slow.

it really does come down to tread and case design.

I think it's also dependent upon the rim width. The newer wider rim profile will make the tires wider.

Thanks for the replies everyone, once my current Specialized Espoir sport 30mm are fully used I think I will upgrade to some 28mm Roubiax Pros or Continental grand prix in 28mm.

Thanks, will look into the Grand Prix, how is the puncture protection on them?

This debate is never going to end, but from personal experience: I've done the same 25-mile RT commute at least 2000 times, on more bikes than I can even guess and on tires from 19mm to 41mm. For about 10 years I recorded my times, and except at the extreme fat tire, low pressure end of the range (2.5 inch knobbies at 35 psi), there was no significant connection between my time and the tires. That wouldn't be true in a time trial, but in real riding, traffic, wind, what I had for breakfast and how I felt that today buried any effect of the bike or tires. My fastest time ever was on an old Bridgestone MB-5 mountain bike with 1.5 inch semi-slicks. On my road bikes now, I have 38mm and 35mm Schwalbes, and I'm not going skinnier.

12%.

Kidding. Best move is to go to a different size and different kind of tire. I'm a fan of "open tubulars" because the cloth casing makes them very smooth. Feels faster. It's not, probably. But they're Italian and give me dreams of Vespas and wine.

Also, Open Paves come in 27.

I'm with you on the open tubs. Challenge Criteriums, to be precise. It's not because I think they"re faster, though they may be. It's because they ride nicer. And the super high thread count makes them highly puncture resistant without compromising the quality of the ride.

just fine in my experience, you can get more protection but you get a heavier tire

I looked at these. Compeitive Cyclist has a nice write up. They mention the non-vulcanized rubber is prone to flats. Challenge adjusts for this by including a puncture resistant strip between the rubber and the threads.