It's a difficult thing to balance as increasing/decreasing training and building speeds also affects the way AI is producing gold/culture/science.
Also, units training speed is also affected by mapsize and handicap: which ones are you using?
Finally, if it's just 1 city where you can build 5 archers in one turn it might be caused by different factors like buildings, hammers overflow or something else. Does it happen in ALL your cities? I've don't think I've ever built 5 archers in one turn on Epic in a normal game.
I've often thought Archers specifically were a bit too cheap. I think a good first step would be to remove Monarchy's bonus to train units in general. Monarchy currently gives +50% training of Archer, Longbowman, Knight, and Noble Knight. I don't think civics should be giving bonuses to train specific unit types. Unit combat classes would be a little more acceptable.