I imported a contact from followupthen.com (VCARD format) to eM Client.
Opening that contact with about 100 mail adresses take up to 10 seconds, before editor is shown completly and ready for actions.
You can test it your own with the contact vcard file from: https://www.followupthen.com/FollowUpThen.vcf
Have you tried closing eMC and restarting?
Have you tried rebooting your PC?
Have you tried completely suspending your VPN and Anti-Virus apps?
Are you running the current version of eMC (V8)?