If you've carried out a speed test (the eero app speed test only checks the speed at the gateway eero, so head back and do the test in the article link above) and the results came back showing that everything is okay (or better), then it may be that the device you're experiencing slow speeds with isn't working properly.
Check that the the devices is on the same Wi-Fi network as the phone you did the test with
Double check that the device that's going slowly has been updated with the latest software and/or drivers if appropriate
Try using another device in the same location, using the same Wi-Fi network, to see how it performs, that should tell you if it's a signal issue or a device issue.
If the 'slow' device is portable, see how it performs next to the router.
It will either:
Continue to work slowly, which would definitely tell you that the device has a significant defect
Work faster than before, which would tell you that the issue can be fixed by providing better network coverage - Check out our article about mesh Wi-Fi for more info