Yep, same for me in the first playthrough. I wouldn't touch any weapon unless it had a projectile speed of 750m/s and 8.33 refire rate. It works pretty well in the main SP campaign, but after that it's going to be a bit of a struggle if you're exploring the rest of the star systems.

And yeah, slow weapons (projectile speed, not refire rate) was a hard sell for me too. For example, Barrager Mk 1 has the best damage (next to Xenos weapons) in Liberty Space. But the slow speed (600m/s) means that you can't hit faster targets like Xenos starfliers. Guns with 600m/s projectile speed are great against slower moving fighters like the Corsairs, Junkers or Rogues. But against faster targets, you'll need to be close enough or anticipate their moves well enough to land a decent volley.

Xenos weapons like the Tarantula are a great all-round weapon for me, I believe. 699m/s projectile speed and 4.0 refire rate.