Tried deleting the gmail account several times; deleting the Mail app a couple of times; ensuring iOS version is the latest; ensuring location setting okay and time zone correct; ensuring no VPN exists; reset network settings; connecting to the internet via Wi-Fi, mobile data and personal hotspot.
My other mail account (BT) seems a bit quicker but it is a very small account so I’m not sure if that’s relevant.
My wife has an iPhone and her Mail app, also using gmail seems okay. I sent a mail to her and she got it straight away.
Even re-installing the gmail account into the Mail app goes really slowly.
I have now downloaded the gmail app as a work round and that works fine.