Mail is causing my internet download speeds to be very slow. Turning Mail off brings speeds back to normal. Is there a problem with Mail? I am running 10.14.6 (18G103) on a 13" MacBook Pro (mid-2014).

Open the mail activity window available within Mail, and see if this gets busy with something. Mail can sometimes get confused, and try to re-download stuff.

DNS translation errors can cause a ~30 second delay in all network activity. There’ll be almost no network activity, and a DNS translation error—as opposed to a DNS server error that can fail over to a second DNS server, there’s a reason most folks have two DNS servers listed—will effectively block all access as the IP address of the remote server cannot be resolved. There’s no identified server available to connect to.

DNS errors will delay or will prevent all network activity, other than the (rare) cases where a remote IP address is already known, or when there’s some other means to identify the remote server, such as Bonjour-based connections on a local network.

When the Mail app (or any similar email app) attempts to send or receive email, it must first use DNS to determine the location of the email server. If there is something amiss with the DNS settings on your Mac, could be one reason why you are seeing a slow down.

By default, your Mac notebook will get its DNS servers info directly from your home's Internet router. In turn, the router will get these from your ISP. One option is to change these default DNS servers to another third-party set. Two very popular ones are Google and OpenDNS. I suggest that you try one or the other to see if it makes any difference.