I used a destroyer for a long time, now level 20+ with my Vanguard.

After we captured a cruiser from the enemy, I transferred command and man the enemy shred me to pieces. Where the destroyer can stand there just turning around in a sea of enemy gunboats to slowly use up and regenerate all shields, they can kill the cruiser.

Also weapons fire much slower somehow?

I did manage to kill a battleship (veteran) with my destroyer fleet (5 ships), wonder if I can capture one they seem stronger. :)

Hmm, not sure, there are different ships, some worse, some better. In SPG2, we had people finishing the campaign in TAURUS, which was the starting ship. It's a new ship, maybe you just need some time to get a hang of it and find a right build (loadout, enhancements...)

Not sure about weapons rate of fire. I'll check it out.

There is no weapon rate of fire as a parameter in game. Some ships are more focused on shields regeneration so they should be played a little different. There is also turret placement that is slightly different on each ship. Sigma destroyer has the best turret placement in terms of firing arcs and coverage.

One thing popped into my mind. When you got new cruisier, did you buy max turrets on each turret slot (Loadout > Configure turret)? Because default is 1 turret per slot.

Assuming equal equipment and enhancements, the cruisers have about twice the shield and about 1,5 the shield regeneration of a destroyer. However, the ships are not created equal, there are differences in shield capacity, refill, weapon energy (which, when drained, would slow the rate of fire to a crawl) and firing arcs.

In general, the cruiser is a definite upgrade, but it would probably help to know which destroyer and which cruiser you were using.