It's not just about speed, it's about distance as well. Do you plan to bomb out casts on big water? Or fish smaller streams in close?
A rod may say it's rated for a line weight. Think about that, it's a true statement. Weight as in ounces, not a rating. But 40 feet of line weighs more than 20 feet of line does. So the rod is designed for a weight of line, or a load, not a line rating... The old standard is that the rod should be rated for the weight of 30 feet of fly line off the tip of the rod (not including leader). And the weight of the line should conform to AFFTA standards. But both rod and line manufacturers have really deviated. It's the wild wild west out there now.
i.e. a rod rated a 5 wt, means a rod designed for 30 feet of 5 wt line. Or 40 ft of 4 wt line. Or 50 ft of 3 wt line. Or 20 ft of 6 wt line. Or 10 ft of 7 wt line. You get the drift.
And then what happened, is rod manufacturers went faster, faster, faster to win distance casting competitions (i.e. they labeled a 6 wt rod as a 5 wt, so when you cast it with 5 wt line, it does better at distance). Line manufacturers followed suit by making the lines heavier and heavier to perform better at regular distances on "today's fast action rods" (they labeled a 6 wt line as a 5 wt to perform better on your 6 wt rod that's labeled as a 5 wt). And then take it another step, the rod manufacturers will start to make 7 wts and call them 5 wts, it's an arms race. Round and round we go. When you go to a store and test cast rods to buy, do you mess with comparing how they cast at 10 ft, or do you bomb them out and see what the rods have at distance? So what do you think the rod guys do? But lines do the opposite, you throw that bad boy on there and go out to the stream, fish at normal distance, and say "this works better than my old line" on my rod. Marketing.
Bottom line? Do NOT be afraid to overline or underline a rod to fit your intended purpose. The ratings are a starting point. That's it. Do not get caught up in this "the rod says 5 wt so I have to put on a line that says 5 wt too." Another manufacturer could call that rod anywhere between a 3 wt and a 7 wt. lol. And the line that says 5 wt could be anywhere between about a 4 wt and an 8 wt as well!
Want to slow down a rod and make it perform better at close range? Go up a line weight or two. Want to speed up a rod and make it perform better at distance? Go down a line weight or two. Understand that to get better performance at distance, you sacrifice some at close range, and vise versa.
I run as much as 3 line weights off on some of my rods. And I vary which line I put on em too, depending on the water I'm fishing that day. That's what multiple reels to rotate around multiple rods are for.