The speeds you are getting on the Tablet T-Mobile One plan are normal (Unlimited 3G Smartphone Mobile Hotspot (up to 512 kbps). When you were on a prepaid plan, you more than likely had an allotment of high-speed and once that high-speed ran out, you would be limited to 3G speeds.
If you want, you can add the One Plus feature ($20) for the tablet and get unlimited high-speed data on the tablet.
I'm on one plus plan and my iPad pro hotspot is also slow as mollases compared to my Samsung s9. Same thing with my Note 8.... Slow as can be. For some reason only the s9 is working as it should for hotspot. All devices are on the OnePlus plan.