One of the weird things about 5th Edition is Cantrips scale with Character Level, but your Martial Attacks do not. So by multi-classing, you're Cantrips are going to be better off.
Consider a Paladin 3/Warlock 2 vs. a Paladin 5. The P/W can use Eldritch Blast and do 2 Rays with each doing 1d10 + Cha, and the pure Paladin can do two attacks with 2d6 + Str (assuming Greatsword). So its pretty comparable. But if the pure Paladin is using a Sword & Shield then its down to x2 1d8 + Str, in which the P/W's Eldritch Blast pulls ahead (and the P/W can still use a Shield).
Honestly, unless you are using a Greatsword and the Great Weapon Mastery Feat, the delay of the 2nd attack isn't that big of a deal. In fact, you can compensate nicely by using Green Flame Blade (which is another Cantrip that scales with Character Level). The only thing you really miss out on is the chance for multiple Smites in a single round.