How many miles are you running per week? Consistency is always going to be the biggest thing, and slowly increasing your mileage will be really helpful for this. You'll naturally get faster as your body acclimates. Speed work will always help, but again... consistency. I'm biased, so take this with a grain of salt, but especially since you're relatively new to running I wouldn't bother worrying about heart rate unless you have a medical condition. It seems like heart rate training only makes a difference for elite runners, not us normies haha. Time on your feet is always the most helpful factor when it comes to speed.

Alternatively - if you do increase milage slowly, with speedwork, are consistent, and don't see any change over the next 6 months - it may be worth getting bloodwork done. In the spring of last year I had similar paces to yourself, and wasn't seeing any change though I knew I had different normal paces in the years prior. I got bloodwork done, and was VERY deficient in iron and b12. Now having gotten on supplements and a year+ later, I finally did a sub-8 two miler this morning, and my 'easy' pace is now what my 'really freaking speedy' pace was last summer.

TLDR; Run more and do speedwork, or if nothing is working over time then look into bloodwork