@Bedscrunchie
Hello,
Please help verify more details and we will try to figure it out ASAP:
1. What is the current network topology/layout? Are all Deco units Deco M5s?
2. Who is your ISP and what kind of modem you are using? Do you mean the wireless speed can reach 150M/150M when all three Decos are working as Access Points, may I know how did you connect all these three to your network? Are they connected to the archer 1700 router by Ethernet cables or the Decos are connected to each other?
3. Are you still using the archer 1700 router when the main Deco is working as a router now? What if you remove this archer 1700 router and connect the main Deco to the modem directly, how about the speed you will get?
Additionally, please refer to the below troubleshooting to check the QoS settings on the Deco app:
https://www.tp-link.com/en/support/faq/2429
After that, please run the speed test again and check whether it improves.
Good days.