I have a Matrix 26" and a Hammerhead (Smooth Hound frame). I don't get either near 30 mph often due to feek and weebleness.
My Matrix (2007) is more mountain bike than roadie - bars are set low and frame geometry puts your weight forrard, so road vision and comfort is slightly compromised - but they make great urban bikes because suspension, larger wheels and tires work well with bad surfaces. They double as capable off-road singletrack weekenders but Dahon bill the Matrix as an urban tool that's dirt capable.
I can top the Matrix out around 28 mph occasionally but I'm not a powerful rider. The 2008 Matrix has a redesigned frame, but similar componentry and ethos. It will fit in your Civic.
The Hammerhead is a bonkers lightweight sprint road bike. Skinny tyres and wheels accelerate extremely well but you feel every road imperfection through it. It 'folds' flat for stowage. Smooth Hound is a more civilised touring version. 2008 versions are lighter and 'break-apart' for stowage.
I'm using the Matrix more often in town now, it seems more rugged and stable in fast traffic. Capable front suspension eats potholes and kerb jumps but I don't get the bike onto the largest chainring much - hilly area, feakly weeble, so I slog along at 14 mph on the Matrix for a 13 mile commute. It's a lot of fun off tarmac though - with a tyre change from the urban Conti Sport Contact smoothies. I wouldn't want to carry it anywhere or commute with it on public transport - although people do.
Components are good MTB spec and low maintenance/easily upgradeable on the Matrix - SRAM 7.0 rear cog swapper, Shimano Hyperglide rear cassette, sealed bottom bracket, Shimano hubs, FSA crankset, WTB rims and Shimano cable disk brakes, and if you shop around you may pick up a discount 2007 bike at 2/3rds list. I'm not a fast rider, but surprised myself on the local trail at the weekend, overtaking the locals on their MTBs. The Matrix is reasonably light (12.5 kilos) for an MTB at this price, and terrific as an urban bike. It's much more comfortable than the Hammerhead, so I ride it more.
A suspension seatpost and a gel saddle work well for trail riding (cheap upgrade) and you can put the lightweight saddle and seatpost back on for road riding.
If I could keep only one it would be the Matrix - it's a smooth runner, works offroad, and has low maintenance, but the manouverable 20" Hammerhead is a blast to ride in traffic and has upmarket components too.
Whatever you get - look at the crankset to see if the front largest chainring is unscrewable and therefore easily change-able for a larger one. On some of the cheaper bikes it's riveted together, and a front chainring change may then mean a crankset-ectomy.
You may not be able to fit a a larger chainring on a 20" bike as Jur says, - but that's outside my knowledge.
All bikes are compromises- if you want a full-on road bike you may end up with that, skinny tyres and all , and used ones are cheap - and fast.
BUT ...having a good all-rounder gives you more versatility and maybe some trail capability at the possible expense of top-speed. For apartment living, road commuting and weekend trailster, you could do worse than the 26" Dahons. If you still need more gear inches, the replaceable large chainring is an easy swap-out though you may then need a longer chain, and possibly a front dérailleur swap.
2007 Matrix
