What version of VMWare tools do you have installed on the parent image?
Were VMWare tools installed before Horizon Agent?
Are you using linked clones?
What antivirus is installed on the parent VM?
Are you using persona management or UEM?
What hardware version is your parent VM?
Do you have "Wait for the network" GPO set?
Are you using Blast or PCoIP?
What client are you using to connect? Is it 4.8 or newer?
Is screen DMA enabled or disabled?