Issue receiving web form emails

I’ve updated a staging copy of a site (on a subdomain of the live site) from Perch 3.0.8 to 3.0.10. The form app was already on latest version. When I send a test web form, replacing my client’s email address with mine in the admin as the send to address, the email takes a long time to come through.

Before this upgrade routine test form email checks to the same address have always come through quickly. If I do the same test on the live site that has not yet been upgraded the email comes through straight-away.

I contacted the host to see if anything has changed there. They are not aware of any changes that would cause this but suggested I switch to SMTP authentication.

I saw a recent thread on the forum https://forum.grabaperch.com/forum/09-20-2017-perch-forms-not-being-emailed with some similarities. Can I rule out that anything in Perch has changed that could cause this issue?

Many thanks