Yes. More Civs = Faster research generally, but this effect is more pronounced the higher you go.
Is it harder or easier?
Probably easier. There are more AIs running around, more distractions for a potential superpower AI. That said, these games can be a lot of fun.
If you can resist the temptation (exploit) of pointless dogpiling that so many Civvers like to do (ie: signing up everyone and their dog against your 1 enemy), and instead concentrate on alliances that make sense to you (without sacrificing anything), you can end up with very interesting Cold War style situations where you are part of a loose coalition of Civs who is vaguely in opposition to another coalition of Civs headed by one of your main rivals. It is also possible to have more than 2 coalitions in your map. Maybe a 3rd or 4th AI power has his own.
Because the alliance system in Civ3 doesn't recognize multiple parties, this is the closest most of us can get to recreating a Cold War style scenario in our epic games.
I would say that as a general rule, if you play on large maps with 16 Civs, or even larger maps with 24 or 31 Civs, you would be looking to spending more time in the diplomacy screen forming deals, and thinking about the effects of your actions very carefully. It takes a level of patience and clarity to be able to handle so many neighbours running around. But the payoff is great.