It sounds like you’re describing a Zone 2 run, and you’ve gotten strong enough / fast enough that there’s a pretty substantial neuromuscular load in that heart rate range, as you said. It might be useful to look at other metrics to assess the load – if you were able to cover 30% more distance in the same time and at the same heart rate, there might be more stress on your muscles & joints than you’re used to, even thought the aerobic element is similar. Consider easing off on the pace, or perhaps doing some fartleks spice it up.
As for the “sluggishness” – that might be a reflection of substantial accumulated fatigue over many weeks or months. Trying to get rid of that sluggishness by running faster could be counterproductive. Taking an easy week, cutting volume by 50%, might help you absorb the training and feel more refreshed.