A little longer, a lot harder

Today I increased the hard paced interval length from 1 to 1:30, but decreased the number of rep from 8 to 6, hence I did 6x1:30x1. Here's my pace per split:

1: 5:45, Max HR 167
2: 6:00, Max HR 175
3: 6:27, Max HR 176
4: 6:37, Max HR 177
5: 7:14, Max HR 174
6: 7:19, Max HR 175

Clearly quickly falling apart. Everything felt hard: stiff legs, no power, heart can't take it... Probably felt the worst of all the sessions since we started.

I think next time I might try to start with a 6:30 pace in the hope that I can maintain it. Not sure what's best from a training perspective: go all out but decrease performance or try to find a level that can be maintained?