Last Viewer with good performance for me was version 1.20.17.
After that all viewers including RCs are much more lagging. Textures and Prims need more time to load. Walking starts with delay.
When more avatars are on a sim even walking against walls or sinking in ground.
Don't know what's the problem with newer viewers, I just hope viewer 1.20.17 will continue to be useable.
This article will show you how to get detailed information about the performance of your computer and the Second Life world Statistics Bar Guide
Please see the Meta-Issue: Textures loading worse in 1.21/1.22 VWR-8503 and linked issues and add your comment and vote to the appropriate issue.
For account-specific help please post a Ticket with Support
Thank you : )