That ship is astonishingly slow for IF. It's moving at base-level Ion speeds. If your enemy has even ion or magneto-plasma tech, they're going to kite you to death. Also, your range is at least 3x too large. Boost engine size and massively increase engine boost percentage. At least 110%. Your beam ships need to be as fast as possible to close on the enemy.
How fast should they be going? I've tried the same engines boosted to 150% and 200% but they get such smeg fuel economy that they can hardly make it out of the Sol jumpgate.
In a recent game, I had an NPR that was ahead of me in engine tech, they had magneto plasma, and were doing 8150km/s, and on Ion engines I had built mine to do 8600, which was incredibly fortunate when we met in battle - I could just close in and force a fight.
I always build my beam ships to be twice as fast as any missile ships, since they need speed far more than the missile ships do. Unless I have an opponent that suggests I need even more speed, I usually build Internal Confinement Fusion ships to a speed of 6900km/sec, twice that(13800) for beam ships, By Ion I'm already pushing 4300/8600.
Range of course suffers, but I find warships rarely need more than 40bkm, and often I don't even give them more than 20bkm, they can be escorted by a tanker to the fight if they need it - I don't need their speed until the enemy is actually engaged.
You might have no need for such speeds, but to answer the question of how fast, those are what I consider to be suitable speed for those two engine techs.