Automatic timer reducing water pressure. What can be done?

I have got a small greenhouse set up, positioned uphill from an outdoor faucet. I’ve got a standard garden hose from faucet to greenhouse. Inside, I’ve got poly tube irrigation and fittings arranged to drip my tomatoes and sprinkle my lettuces. This was all flowing great until I added my automatic water timer. Once that was on my flow decreased dramatically and only the drip system worked (and very feebly at that). When I turned off all the drippers, the sprinklers struggled to life.

Without the timer, I had about 15 different outlets but with the timer it can barely maintain three.

I’m wondering if there is another style of automatic timer that won’t drastically affect flow? Ideally not wildly expensive. Other than that, I’m not sure what else to do. Would cutting out the hose and having all poly tube help?