PVS BIOS based target devices were slow to boot, with following observations:

• BIOS based target devices often taking tens of minutes to boot successfully, occasionally target devices may fail to boot.
• The network conditions in standard production network were non optimal for PVS boot performance. 
• Additional testing was completed, performed on an isolated network which had optimal conditions, no issue was reproduced.
• Testing was completed also against UEFI based target devices, both on the isolated network with optimal conditions, and on the customers standard production network with non optimal conditions, and in both scenarios UEFI based boot, no issue was reproduced.

Customers must switch to UEFI based target devices.

• Support for all BIOS based target device booting has been deprecated since PVS 2203 release.
• BIOS based target device booting issues are not valid for in-depth investigations with Citrix technical support.

Problem Cause

Support for BIOS in Citrix Provisioning:

• First announced in Citrix Provisioning 2203, BIOS based PVS target device streaming is deprecated in all supported PVS versions now, and all future PVS releases.
  • BIOS based target device booting remained in Citrix Provisioning 2203 to provide customers with a bridge for moving all their PVS target devices to UEFI based booting, while continuing to ensure customers PVS servers were correctly maintained and upgraded to the latest releases and cumulative updates.
• UEFI based target devices have a twenty fold theoretically higher network throughput during the PVS boot phase.
  • In optimal conditions this boot phase is very short, with both UEFI and BIOS this phase would be measured in tens of seconds or less.
  • However in non optimal conditions this difference in potential network throughput becomes clearly observable, with BIOS based target devices boot phase significantly increasing to tens of minutes.
• BIOS based target device booting is more vulnerable to performance issues and is less reliable than UEFI based PVS targets devices when booting during sub optimal network conditions.
  • Sub optimal network conditions can be any of below conditions individually or combined
    • Network latency higher than 1 millisecond
    • Any detectable UDP packet loss.
    • High network saturation, high volumes of network broadcasts.
• As BIOS based booting was deprecated in PVS 2203:
  • BIOS based booting issues are not valid for in-depth investigations with Citrix technical support.
  • There will be no future updates to BIOS based target device booting.

https://docs.citrix.com/en-us/provisioning/2402-ltsr/deprecated-features