List V3 has always been an issue since it's release with lots of customers mentioning its performance issues. For those of you using still using List V3, or are planning to do so, there is a clear message in the London Release notes on this page:

https://docs.servicenow.com/bundle/london-release-notes/page/release-notes/summary/rn-summary-upgrad...

additionally on this page:

https://docs.servicenow.com/bundle/london-platform-administration/page/use/using-lists-v3/reference/...

the advice is:

Reading between the lines, it looks like List V3 is going to be deprecated in a future release. So now is probabaly a good time to start planning how you are going to deal with this if you are currently using List V3.