Instance running very slowly - not sure on best instance type

It can be a problem with the EC2 instance itself being overloaded or it could also be because of network band-width. Are the users connecting to this instance over the internet or from your corporate network over IPSec VPN or DirectConnect? IPSec VPN has a band-width limitation of around 1.25 Gbps. If using DirectConnect, are you using 1Gbps or 10Gbps? If you are using a transit VPC where you have your network firewall devices and you are connecting to the VCPs using VPN, then also you will be constrained to the 1.25 Gbps even if you are using 10 Gbps Direct Connect. It is possible that other applications are hogging the network band-width. For investigating if the EC2 instance itself is overloaded, you can look at the Cloudwatch metrics - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-cloudwatch.html.

Likewise, for checking the performance of your IPSec VPN, you can look at Cloudwatch - https://docs.aws.amazon.com/vpn/latest/s2svpn/monitoring-overview-vpn.html

If you are positive that the network band-width is not the issue, then you need to look at the Cloudwatch metrics for the EC2 machine in question - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-cloudwatch.html and see if it is running high on resources. In that case, you may want to go with a more powerful box.