So many variables to consider. First.. have you directly connected a computer to your ISPs modem to confirm you are actually getting the new higher speed? If you are, next step would be rebooting your Nest Wifi to see if it needed to refresh itself. If a reboot doesn't work...a factory reset may be in order.
I had a speed issue last year, and ended up factory resetting , as one of the points was causing a bottleneck.
