It's quite possible; light frigates pretty much have the sole role of chasing down support cruisers and frigates, so they may have some elastic properties to keep kiting ineffective against them.  Other than that, there's also the possibility that your units were being slowed down because of fleet behavior.  I know that in many games when you need units to move at maximum speed, you break up any groups you have and give individual orders, otherwise cohesion behavior can screw them up and get them killed.  I typically don't use fleets for this reason; I'd rather do the cohesion micromanagement than micromanage around the cohesion issues.

There's no research or artifact that affects movement speed.  TEC have a research for capital ship turn speed, and a special ability for heavy cruisers, but otherwise you can't improve base unit speeds.

Research does not apply to what the OP is saying.  it was a neutral gravity well.  It would be the neutral malitia defending it using TEC style ships.

Since pirates use the same style ships, what are their specs?

Are the ships slowing down when they're under fire?