Forget weight, gearing is what’s most important. All the major component manufacturers offer cassettes with a wider range of cogs than comes standard on most bikes. Sram even makes a derailleur in its Apex line that can accommodate a super-easy 34-tooth cog. Count the number of teeth on your largest cog; if you have fewer than 25 and you struggle with maintaining a high cadence when climbing, you will surely benefit by upgrading to a cassette with 27 or more teeth on the largest cog (cassettes typically cost around $100). Conversely, if you ride in an area with few hills, consider a cassette with tighter spacing, like an 11–23, for smoother shifting performance. Also consider adding a cadence meter to your bike, as gear choices are often informed by cadence changes.
Pro tip: Before you buy a new cassette, take note of your favorite gear combination. Count the teeth on that “sweet spot” cog and then make sure that your new cassette includes one with that exact number of teeth. While it is tempting to go for the widest possible range, those cassettes skip over popular cog sizes like the 16-tooth in order to make the jump. If that’s your favorite cog and you don’t mind coasting when you top 35 mph, consider a 12–28 instead.