I think it's safe to say that Borderland Ruins Secure is the worst Frontline map.
But why?
What can be done to improve it?
TL: DR - Remove the invincibility buff after respawning or make it 1400 Tactical Rating needed to win a match. 1600 currently is too much.
Long Answer: The first thing that comes to mind for me is that match duration is longest of all four maps, on average. It's the only map where matches can last all 20 minutes. The main culprit for this is the invincibility you get after leaving the spawn area. People aren't bothering going for more than 3 bases since if you go to another team's home base, you will just die due to enemy players having invincibility.
The 1600 total tactical rating (TR) needed to win a match takes into account that teams may have 4 or more bases throughout the match. Unlike the bases on the Shatter map, which only grant 2 TR over time, for a total of 6 TR if you hold all 3, the TR you get over time from bases on Secure increases exponentially. Starting at 1 TR for holding 1 base and up to 32 when you manage to keep all 6. But, again, trying to go for 4 or more bases is too risky because of the invincibility you get after respawning.
If you want to make the map more interesting, you should remove the invincibility buff on Secure but keep it on the other maps. If the invincibility buff must stay, you should reduce the amount of TR needed to win a match from 1600 to 1400 (maybe even lower) or generally increase TR gain from various objectives on the map. Be it bases, drones/node and K.Os.
That's my take. I would like to hear other suggestions from people.
Edit: Just to spell it out, here's how bases work on the maps that have them.
Secure:
1 base = 1 TR over time
2 bases = 2 TR over time
3 bases = 4 TR over time
4 bases = 8 TR over time
5 bases = 16 TR over time
6 bases = 32 TR over time
Shatter:
1 base = 2 TR over time
2 bases = 4 TR over time
3 bases = 6 TR over time.
That should give you an idea as to why matches lasts so long on Secure with the invincibility in place.